They were written by pretty much one person, who also fixes bugs, maintains the manual and help out the maintainer -- me -- with any tasks needed. We usually start the release notes for the next major version right after we released a major version, and try to keep up with what happens.
Upcoming Krita 5.1, that what's currently in the master branch on invent.kde.org, already has a bunch of new stuff...
(meta: replying to a comment is disabled for a short period, which increases depending on depth of discussion or something like that, in an attempt to reduce unproductive back-and-forth argument)
"We usually start the release notes for the next major version right after we released a major version, and try to keep up with what happens" That is a best practice right there, and I've long been an advocate of 'continuous documentation' because it's actually less work per unit of quality over time.
