I wonder if development has actually accelerated, or if this is just a change in the approach to the release/versioning process
Both.
Development has increased, but you should use your comparison from the last 2.6 release.
It stayed on 2.6.y for 8 years - that was where it got stable enough that there wasn’t some major milestone to use as a new marker for its update number
There are cool new features, but if it followed the old versioning scheme, we’d still be on 2.6 because it hasn’t (intentionally) broken the API between the kernel and userspace
Since version 4.0 the version numbers have nothing to do with changes and are strictly time based. Linux 5.0 happened after Linux 4.20 because Linus "ran out of hands and toes to count on", same thing with 6.0 after 5.19
Edit: more seriously it’s been since 3.0 after being on 2.6 forever
there are no special landmark
features or incompatibilities related to the version number change,
it's simply a way to drop an inconvenient numbering system
It used to only get bumped after a major new feature update, but it was stable enough at 2.6 that it got stuck there for 8 years, so he switched to a different update number