From the godfather Himself:

I’d like to point out (yet again) that we don’t do feature-based releases, and that “5.0” doesn’t mean anything more than that the 4.x numbers started getting big enough that I ran out of fingers and toes.

https://lwn.net/Articles/781206/

From kernel.org:

Does the major version number (4.x vs 5.x) mean anything?

No. The major version number is incremented when the number after the dot starts looking “too big.” There is literally no other reason.

https://www.kernel.org/releases.html#does-the-major-version-number-4-x-vs-5-x-mean-anything