Back in 2020, Discourse switched from the .js.es6 extension to .js. Over the years, the vast majority of themes and plugins have switched to the new extension, which provides much better compatibility with modern JavaScript tooling.
We are now formally deprecating the use of .js.es6, and will remove support after the next ESR release.
If you see a deprecation message for .js.es6 files, you simply need to rename them to .js. That’s it. No need to change the content of the files.