Today, a newly opened topic reply has a new "Topic" dropdown at the top of the reply, with only one entry, "Reply as linked Topic". As well, this message box I'm typing a new topic within has a similar dropdown, but it's sole entry is "Create new personal message", despite being titled properly as "Create topic". Huh?
Is this new, or have I been sleeping recently?
I did observe the same; see point #2 in Redesigned post composer - #2 by sterretje (from 2 weeks ago). The new composer is now implemented for all users and what you did observe might be a result of that change.
But I might have been sleeping as well in the past 
Experts at work.
... and when editing an old reply, a popup asks, "Reason for editing this reply?"
I've seen that.
There has always been the option to add a "comment" when editing a post; it's now more prominent.
The "comment" shows when you check the edit history of a post.
Continuing the discussion from More forum changes?:
The menu has always been there, but it was previously only a icon:
This was redesigned recently:
It is separate. The only change introduced by the composer redesign was the removal of the border. Here is how it looked before the composer redesign:
Previously the post edit reason field was opened by clicking a ⓘ icon. I'm not surprised if you never noticed it, as it was quite subtle. The new edit reason UI design was part of the same set of changes that made the post type menu more visible.
