ZDoom

There are ways to do this now through WorldLinePreActivated.

I've thought of a way to do this easily. I was working on a "WorldLineActivated" event for a different purpose, but it would also allow for handling this. The way I'm writing it at the moment, it's fired on line activation before executing the special but right after checking if the line has a lock, so I can pass the fact that it's locked as a parameter to the event ("ActivatedLine" and "IsLocked" would be added to the WorldEvent struct for this). Linedefs already have the "locknumber" variable exported so it should be trivial to implement what you need.

I'm testing it right now to make sure things work properly.

Edit: A terrible misunderstanding happened. Seems like lock checking isn't done where I expected. This will require different handling.

The way I see it, it'd make more sense as a property of the status bar (since it's UI stuff). Alternatively, it could be a MAPINFO setting, but I like this less.

I don't see much point in mixing and matching "locked door" feedback within the same map. Consistency demands this kind of things to be a global property.

I can think of some ways to do this. It could either be added as a LOCKDEFS extension (like, for example a "LockedSpecial" parameter for executing an action special), or as some virtual static function on the Key class that can be set per-subclass that gets called when trying to open a locked door that needs said key.

The flashing key thing is not currently possible without a custom implementation in either ZScript or ACS.

I am not sure if this should be an engine feature or not, though, as I see merits both to the "DIY" mindset as well as actually having it available. So I am going to leave this open for Graf to decide.

Audio: locked door sound is defined in SNDINFO as misc/keytry. For example, it is used in Beautiful Doom. So it is possible to make a mod with the sound that you like.
Video: don't know about flashing a key, but it is possible to set appropriate color for text message that appears when you try to open locked door. Example: Maybe this is sufficient.

Heyo, this is my first time making a feature suggestion here. So I hope I don't waste your time too much.

I've noticed a lot of TC's and projects regarding the PS1/Doom64 mods and ports seem to be missing one thing. PS1 and Doom64 flashed the key I was missing from the door via audio visual feedback. The audio being the standard item pickup sound repeating at least three times as well as flashing the key in the hud. I haven't seen modders try to use ACS to make it a thing and I don't know if it is. The only project I know that has it is Doom64 EX which is completely different port and tries to be as faithful as possible to the original Doom64. All of which seems like old news by now. Sorry this is not some extreme mind-blowing or cool idea. It's pretty insignificant. I just thought it might be something to suggest since I heard you guys added some kind lighting system for maps defined with Doom64 lighting or something.