This post is about unwanted files ending up in git. So if you don't use git and you collaborate by swapping floppy disks or whatever, you won't have this problem. (But you'll have other problems, of course)
What project is using .gitignore but not git?
For one thing, many AI code helpers use the contents of .gitignore as a suggestion for files to not include in their context or changes.
My bad, I misinterpreted the proposal.
Lots of non-git tools use them. Some that come to mind are ripgrep (by default ignores .gitignore'd files), some editors (greying them out in the file selector, not searching them, etc), rustic (not backing them up), probably more.