Is your feature request related to a problem? Please describe.
As far as I can tell, everything custom untriggers can do could be done directly in custom trigger by returning false as long as you know if the trigger is already active. The Wiki page mentions "Automatic" and "Custom" Hide options, but I only see "Custom" and "Timed", not "Automatic". I'm on 4.1.4 (Wrath). I understand that lots of existing WAs might rely on untriggers, so compatibility would have to be maintained.
https://github.com/WeakAuras/WeakAuras2/wiki/Triggers-and-Untriggers

Describe the solution you'd like
Hide and don't use Custom Untrigger by default

Describe alternatives you've considered
Default custom untrigger to

function ()
  return true
end

or how documentation describes it.

Additional context

There's no issue at all. Untriggers have a place, and no matter whether you think so or not.

Untriggers can be handled in the trigger function in "Status" Triggers only, and for those Triggers a blank untrigger function does return true.

I don't know why you came to the conclusion that untriggers should be deprecated, but that would be a wrong conclusion.

Thank you, my main problem was that I was using an Event trigger instead of a Status trigger. I started using WA yesterday and wondered why my auras didn't hide when returning false/nil.

However, I still believe that untriggers are unnecessary for the reasons detailed in the OP, but the severity is far lower.

For Event Triggers they're vital.

If you wanna chat about using custom triggers (or more broadly just using WA in interesting ways) without the context of a Bug/Suggestion ticket, join the Discord.
https://discord.gg/weakauras