Please list some example names of these files.
I've used the software for a very long time and never noticed an excess of .lsl files.
I have seen a build up of .lst files, which can be deleted.
Deleting .lsl files will prevent drawings which used those files from displaying properly; at least that's the way the documentation says they work. I've been disappointed with PTC on line fonts so never defined my own; Computervision was better and even included tree shapes or any shape you selected as line font patterns.That was 30 years ago.
If they are .lsl files, open a few up and see what the contents are like.