Recently, the #collaboration section was re-opened (). The following explanation was given by @Hooksmith:
All categories are re-opened for posting, however threads in #collaboration:recruitment auto-lock, similar to the #bulletin-board. However, this is not how threads originally were behaved prior to closing (e.g.,: Recruitment - Developer Forum | Roblox - this is a sorted list of the threads by replies, just for the purpose of demonstrating that replies used to be allowed).
I assume this is a bug, because calls for the return of #collaboration (in the context of the original thread the above quote was pulled from) were done citing the way the category was used in the past. Restoring it (as seems to be the intent, even if temporarily) would imply returning it in full. As a result, this most likely seems to be unintended behavior.
In the event that it is not unintended behavior, I would shamelessly like to take the space to briefly say that replies to threads in #collaboration:recruitment used to be very helpful to people looking for jobs and for people trying to find developers. Users could ask questions for the benefit of all (e.g., “You say you need a bunch of weapons designed, but how many and in what art style?”) and that answer could be shared by everyone looking at the thread. As a result, an ambiguous thread was less likely to die out, so long as an answer was put forth. Those questions saved a lot of people making recruitment threads for the first time, who probably didn’t know they were leaving out crucial details. At times too, the inability to answer those questions could also be useful to developers in assessing multiple potentially promising threads.