So what's your definition of tunneling

And yet, most survivors STILL think of DS as an anti tunnel perk when it has no consecutive dying state activation requirement and instead just has a timer.

To me, tunnelling is when you hyperfocus one specific Survivor on purpose, repeatedly, until they're dead, even when other viable targets are possible. You just pick one Survivor, and focus on them so all they get to do is be chased and end up hooked. By this definition it follows what others here are saying.

It MUST be purposeful, intended, and done with the objective of getting one person out as fast as possible to tilt the game in your favor. That IS what tunnelling is. That IS the problematic kind of tunnelling. Thankfully, this sort of tunnelling is very rare! Most of the time what people call tunnelling is not this, even if it might feel like it.

Examples of True Hard Tunnelling:

• The Dredge decides to ONLY chase Laurie all round until she dies, because the player does not like Laurie (this happened to me last night, I was the Laurie).
• The Nightmare catches Cheryl, hooks her, and then leaves. The Nightmare then, on later returning to check the hook, finds Meg unhooking Cheryl and chooses to deliberately target Cheryl instead of Meg.
• The Huntress sees Yui and Jake healing under hook, Jake has recently gotten off hook. The Huntress decides to aim her axe at Jake instead of Yui, and refuses to let Jake get healed by throwing axes specifically at him whenever he tries to heal or is healed.
• The Nurse purposefully keeps returning to the same hook with Blink because she can, just to down Taurie when she is taken off the hook and harass her.
• The Shape is running Tombstone Piece and REALLY wants to get a Mori on Vittorio specifically, so he chooses to stalk everyone but only chase Vittorio. Vittorio is an excellent Looper, however, and manages to run the Shape for two gens. This angers the Shape, he catches Vittorio and Moris him, but then gives up and lets the rest of the team go after getting his Mori.

Tunnelling can sometimes be done to punish sweaty or griefing behavior, but it's your call on if that is justified or not. To me if you're doing it to punish bad behavior, it's understandable but still a mean thing to do to someone. Examples:

• Claudette brings a map offering that is heavily Survivor biased with the purpose of hoping for an M1 Killer, like Trapper, to give the player an advantage or exploit a certain build. Unfortunately the Killer is The Nemesis who was burned while playing The Pig a few rounds ago by this sort of shenanigan, and decides to force Claudette out as punishment.
• The Oni notices that Ace is being a real jerk to their team - sandbagging, not doing gens, trying to force their attention onto other Survs, just really griefy behavior. To teach Ace a lesson and to let the others get points, the Oni decides to tunnel out Ace and let the other three go.

Soft, tactical, and accidental tunnelling can also occur. This is the difference to me between hard tunnelling and tactical play, there MUST be intent there for it to be problematic. If it's not intentional, is a tactical play, or is an accident, it's okay! Examples:

• The Houndmaster is patrolling hooks when she notices David pull Dwight off the hook not far away. To pressure the unhook, she decides to sic Snug on them, and Snug ends up biting Dwight instead of David, so she simply takes the free down and rehooks Dwight. She then ignores Dwight the rest of the round.
• The Wraith has just downed Leon, but knows Jill has a Flashlight and Champion of Light. The Wraith did just chase Jill and hook her earlier, but knows Jill is behind a rock waiting, so the Wraith runs over to the rock to spook Jill away from the downed Leon so he can hook.
• An experienced Killer player is currently playing as The Demogorgon, and doing well. They choose to spread hooks by hooking Lara, then the unhooking Baermar, then finding Lara to hook her again. That is, they ping-pong hooks. They then start to go for Renato to spread hooks more, but leave Gabriel for last as Lara, then Baermar, then Renato and finally Gabriel fall, resulting in more even hooks, more hooks spread out over time, a 4k, and a more fun experience for everyone.

The following would not, IMHO, be examples of tunnelling because they lack intent, are tactical, are brought on themselves, or are an accident:

• The Legion sees that Zarina and Jane are healing under hook. Legion did just hook Jane, but has activated Feral Frenzy and this is where it led them. So, Legion runs in, hits Jane with their power, hits Zarina, and then leaves (This happened to me in Chaos Shuffle 1.0, I was the Legion, and yes they did accuse me of tunnelling for this).
• The Ghostface is pressuring a hook by stalking Mikaela unhooking Sable, and finishes the Mark on Mikaela. Using his stealth and his haste from Drop-Leg Knife Sheath, he runs in and goes to stab her, but accidentally stabs Sable instead. The Ghostface then sees the opportunity to generate much needed pressure and maybe even down two Survivors at once - Mikaela is exposed and would not do anything risky, and Sable's offhook endurance just got canceled, so he chooses to go after Sable to down her, then downs Mikaela a few seconds after and hooks both.
• The Spirit keeps finding Feng-Min and nobody else because Feng is wearing a brighter skin than her companions, or her companions are hiding. Therefore, the Spirit chases Feng repeatedly because that's the only person she can find.
• Nea has one hook and is feeling very cocky with her Off the Record. She sees that the Knight is chasing Aestri (who is healthy and just got Steve off the hook), but thinks she can do a cool play and runs in to body block with the endurance, despite just getting off the hook a few seconds ago. This annoys the Knight, so he chases and downs her.
• A newer player decides to try the Ghoul for the first time, and realizes he is very good at patrolling. They're even good at hitting Survivors when they do things like unhook without giving them a chance to heal! Oh wow they're having so much fun with this character, they're so glad they started playing DBD, maybe they will even Main the Ghoul! In one round, this baby Ghoul sees Yun-Jin, who just got off the hook, is on a generator he just pressured. So, they innocently decide to go after her again, wow this chase stuff is so fun, and she's injured so she is a good target! They down Yun-Jin and hook her, but she DCs. They are confused but continue the round and 4k. To their shock, the entire group of Survivors yells at them in the endgame chat about something called "tunnelling". The baby Ghoul doesn't understand, what is "tunnelling?" They just wanted to chase and Yun-Jin looked like the best target, are they not supposed to do that?
• Dracula hears whimpering behind a rock while in Bat form and shifts to Vampire Form to see what Survivor it is, because several are injured and he can't tell. Without really thinking, he hits the Orela who just got off the hook instead of another Orela in the same round who is also injured and wearing the same skin. He then hooks the downed Orela, forgetting to check the HUD, and doesn't realize his mistake until he notices the other Orela has no hooks later.

So, it's multifaceted as you can see. To me the key is it HAS to be intentional, it HAS to be for the purpose of not letting one person play the game, and it HAS to be malicious. It's specifically chasing the same person on purpose and ONLY them for the majority of the round with the intent to ruin their game.

tl;dr - if you focus one person all round to ruin a game or punish bad behavior, yep that is tunnelling and sweaty/griefing depending on context. Anything else explainable by an accident, tactical play, using a power as intended, or caused by a Survivor playing badly/getting in the way of someone else is NOT tunnelling.