You can retire a set and choose to keep the individual cards in your inventory or remove all from your inventory at that time. If the set was in the Top 5 of that set all time, it will stay on that list as a retired set. The new buyer can then register all the same cards, create their set, and improve on it with their own additional cards. Then the new set would show up in the Current Finest and All time finest lists. The retired set is not in the current finest listing.
PSA will make back runs if you ask them to, if someone hasn't done it already. You just need 5 cards of a "set" to make the request.
T206 sets with back brands:
https://www.psacard.com/psasetregist.../t206-sets/215
1952 Topps with some retired sets still in the top 5:
https://www.psacard.com/psasetregist...opps-basic/629