PHPStan outlines its backward compatibility promise on these two pages:

• https://phpstan.org/user-guide/backward-compatibility-promise
• https://phpstan.org/developing-extensions/backward-compatibility-promise

The TL;DR is that things like extension APIs, configuration parameters and CLI options will not break unless it's a major version release. And that's the most important thing about semver that PHPStan keeps.

In case of static analysers and language tools like PHPStan or TypeScript, the line between bugfixes/improvements/new features is really blurry and not really practical. In my opinion there's no downside in releasing something in a patch version instead of a minor version. Everyone should have the caret constraint like ^2.1 in their composer.json anyway, meaning their project will upgrade whenever I increase x or y in 2.x.y anyway.

What's important to me instead is the workflow when developing PHPStan. Each new minor version gets a new default branch. Right now we're on 2.1.x. I only create a new branch when I know the development of the feature will take some time.

Some examples:

• I opened 1.11.x to work on error identifiers because I knew it will take me some time to finish them. It took me around 12 months. Thanks to the new branch, I was able to develop and release new 1.10.x versions in parallel.
• I opened 1.12.x to pave the way for PHPStan 2.0 for a long time and still release 1.11.x versions meanwhile.
• I opened 2.0.x to work on 2.0 BC breaks and I was still able to develop and release 1.12.x versions meanwhile.

Another reason why I don't do new branches more often is because there's some overhead work involved in that:

• Properly update GitHub Actions workflows across phpstan-src and phpstan/phpstan
• Update all source links leading to github.com on the phpstan.org website
• Update target branch in various auto-updating workflows in other repositories, like phpstan/phpdoc-parser or my BetterReflection fork
• Update base branches of current wip pull requests in phpstan-src

I hope this answers your question and helps you see through my reasoning.