Pure text copy/paste, often fairly large chunks 1000 - 2000 words.
The only recent change to the site (other than the update on the 29th) was to change the number of consecutive posts a user can make, from 5 to 15.
My most technically adept user says the last update to his Windows system was on the 18th, and the problem didn’t start until after the 29th.
Edit: I went looking for anything that might have to do with content extension filtering, and my site is using the default list of what’s allowed, I haven’t made any changes to that at all.
I then filtered the settings search on “paste” and found this:
Enable automatic HTML to Markdown conversion when pasting text into the composer. (Experimental)
which is enabled by default, so I disabled it. I’ll check with people who use Word to see if that lets them paste as before.