Usually DIRECTORY_SEPARATOR is not needed at all.

Only when PHP paths from conf/OS or comparisons with DIR are done we may need to normalize it to forward slashes .

But all the handling within Moodle should be, exclusively, forward slashes. All OSs understand it. More specifically within Moodle APIs (file, caching...) it never should be used.

https://dev.to/c33s/always-use--as-directory-seperator-in-php-43l7

So a warning explaining the above whenever DIRECTORY_SEPARATOR uses are found... may be a good thing.