We do not autoclose topics on my forum, as we don’t think necroposting is bad. Any discussion can be picked up again, and if it goes off topic we can split it.
Sure, picking a ridiculous date seems like a good stopgap. And I understand that the main usecase would be for limited slow mode.
But ideally forever slowmode should still be available, and this should be reflected in the UI better – something like a “Forever” option after the “Six Months” one would do it for me.
The backend obviously already allows for this, so it would just be a small UI change – a worthwile one, in my opinion.