Just review the definitions directly, where they are actually stored on your system. Probably:
/usr/share/spamassassin/20_freemail.cf
/usr/share/spamassassin/20_freemail_domains.cf
# and possible later versions at
/var/lib/spamassassin/{version}/{repository}/20_freemail.cf
/var/lib/spamassassin/{version}/{repository}/20_freemail_domains.cf
Such handcrafted lists rarely need hand tuning, though. You might rather want to retrieve sensible non-zero (but still small) defaults for these simply by fetching a (still somewhat maintained) updated rule set.
Its just an old, manually compiled list of domains known to have offered free mailboxes in the past. Nothing special and by nature always incomplete. Used for other useful signals in fighting spam, but generally not significantly on its own. A message claiming to be from one such known provider does not mean much in either direction: legitimate mail or trash. Check the other rules incorporating these, though. Sending from a freemail domain while also setting reply-to to something no private or business entity would reasonably do is a strong spam signal, that is why Spamassassin is right to keep the list around.