In this release, the mechanism for highlighting of found words in snippets has been rewritten. In the previous implementation, these words were identified using regular expressions. That approach required the stemmer to be able to reverse the stem back into possible irregular words. After implementing formatting preservation in snippets, formatting and highlighting started to conflict. Now, the highlighting has been made more consistent.
The major version has been increased for this release because the IrregularWordsStemmerInterface has been removed due to being unnecessary. If you didn't have your own stemmer implementation, you can safely update to this version. There are no other backward compatibility breaks with the previous version.
Full Changelog: v2.1.0...v3.0.0