Perhaps related to this?

While this gets squared away by the SublimeLinter folks;

Go get this fork, download or pull the repo. Installing the repo of the author failed for me - so manually getting it and installing was needed. Rename it to SublimeLinter-contrib-proselint.sublime-package and move it to /home/ColonelBuendia/.config/sublime-text/Installed Packages/SublimeLinter-contrib-proselint.sublime-package.

Should work when syntax is plain or md. Also proselint was in my distro's repos now, under python3-proselint which was super convenient.

PRO-TIP: If you download it, it may be a zip of a folder and not a zip of the files - and nothing will work. Move all the files to the root of the zip if so. Let's never discuss how long this took me to notice.

While this gets squared away by the SublimeLinter folks;

I hope this is fixed some day, because this is a very useful package in my work. But thank you for the tip—I can confirm that this fork actually works 👍

And yeah, the author repo doesn't seem to be working, so had to download it manually (and get rid of the folder in the zip :-)

Well... it worked for a while. Then I stupidly did the Upgrade/Overwrite All Packages thing from Package Control, which overwrote the package with the main branch version and now I can't get it to work anymore for some reason... even if I delete the whole Sublime Text folder and install all packages from zero :-/ Everything should be exactly the same, but it doesn't work anymore.

EDIT: Hmm, looks like proselint itself is giving errors... No idea why, because I didn't touch it.

It sounds like the forked version really should be the canonical version.