asking the author(s) to free their code
You can start with alyssa-p-hacker who switched SchemeBBS from a free license back to an open license in commit:
https://github.com/alyssa-p-hacker/SchemeBBS/commit/f5e9a48eb2676e1a0be0387ddcfef7a5a7554a7b#diff-9879d6db96fd29134fc802214163b95a