I just added $USER\AppData\Local\Microsoft\SyncToy\2.0\*.log
to my files to be deleted, but maybe this would be a useful option for others. Microsoft's optional "synctoy" notes its activities very carefully. After a while, one is not very interested in the files which were copied a year ago.
I do see that this might be a silly suggestion. I just did a search in AppData and there are a zillion log files, most of which I never read and maybe one should just add something to a personal list of files to be cleaned.