Can I safely delete sw.old folder on my MacBookPro?
http://www.finkproject.org/ installs in /sw, so this may be a leftover of using that in the past. You might want to check your .bashrc etc to see whether any paths still point to that folder, but it's most probably safe to remove.

What I usually do in such cases it to rename the directory (e.g. sudo mv /sw.old /sw.old-DELETE-IN-SEPT-2019) and then wait a month to see if something breaks.