Nice! It's a pretty self-contained patch, so I was able to add it fairly easily. I left a warning on the post page, though, since I don't think it was otherwise obvious enough what was going on.
it's a good thing that you implemented it without hesitation. To be honest, I haven't faced any challenges trying to quote from locked threads -- maybe that's because I haven't actually tried to.
For the warnings, I'd say - you didn't even have to bother yourself that much; a
biggass padlock emoji on a thread, with the
reply option missing is enough to tell me that I can only read through i.e if it's one of the old, intriguing sagas of the forum before/during lauda's epoch.