IIRC signature ban is somehow common when admin/moderator decide to remove permanent ban, although i no longer remember which member who got such treatment.
Previous only head admins can: theymos and Cyrus but since 2019, global moderators can do it too.
Flow is admins collect proof and make reports to global moderators/ admins who will make further investigation, then make decisions to keep the permanent ban or release it with a softer one.
Posts are reported to moderators who check out the report. If the poster needs to be banned, the moderator sends a ban report up to a global mod or admin. The global mod or admin handles all of the ban reports they get at around the same time. Either the admins or global mods don't need to check every account because they trust the moderators to have already done so, or they check quickly because every such report contains references and links to the plagiarism post and to the source text so checking takes little time.
Global mods were given the ability to ban signatures only yesterday. Do you think we can get through all the appeals in one day? Be patient.