Generally/Ideally things break early in the release cycle, bug reports show up on the mailing lists, and fixes are applied. They are marked with a Fixes tag so as for stable releases to pick them up if relevant.

For basic stuff (as in checking if boards still boot) you have projects like kernelci.org that detect regressions on a huge variety of boards. That said, if you need to support a touchscreen, or say, an uncommon USB device, it's up to you to take care of keeping up with the releases.