To allow conditional compilation into either MV2 or MV3, the source code was fully refactored with many parts written from scratch, which is why the major version is changed. Conditional compilation is certainly a nuisance and I'll probably have to come up with a more modular architecture to make it less spaghetti-like, but it necessitated a rewrite that in turn made new development features possible: modern syntax, UI frameworks, and tooling. The final compiled code is even more efficient due to tree-shaking (dead code elimination), bundling dozens of small scripts into one, and partial minification (function names are kept intact to enable meaningful stack traces). Some old bugs were fixed along the way, some new bugs caused by the rewrite were reported as well and fixed (except #1851) since the new code was published in the beta channel.
This is the last MV2 release for the Chrome web store. In the future an MV2 zip will be attached to each release, which can be installed as an unpacked extension (you can subscribe to receive notifications for new releases in the Watch button on our main project page).
Full Changelog: v1.5.51...v2.0.6
P.S. Not publishing it for Firefox yet because I want to finish polishing the MV3 version for Chrome web store and there are no major bug fixes for Firefox anyway. To help us release sooner, use this zip as a temporary extension in about:debugging page and report the bugs here. Don't forget to export a backup of your styles first.