My understanding is that when vehicles are sold, private party, that the software features remain intact, but that if a car is traded in to Tesla, Tesla CPO will delete free transferrable lifetime supercharging (early) and then sell it without. And of course, for vehicles with nontransferable lifetime supercharging (later), they are in their rights to take it away on transfer. Apparently it was observed last year that cars that all used teslas in the CPO were suddenly missing the free lifetime supercharging function the early ones were sold with. I guess they could do the same with FSD, maybe even EAP and AP.
Acronyms:
FSD= full self driving, i.e. EAP plus auto lane-change and navigate-on-autopilot and future autonomy
EAP= enhanced autopilot, i.e. AP plus the self-parking function
AP= basic autopilot, traffic aware cruise and auto-steering