It's an interesting timeline, isn't it? H&M made their 'stepping back' [to] 'carve out a progressive new role within the institution' statement on Instagram on 8th January 2020, under Trump as you rightly say; and were, after the urgently convened 'Sandringham Summit' on the 13th Jan 2020, given a year to think about it.
About four months into the transition period that QEII thought was to be spent between Canada and the UK, H&M moved to California.
Biden was inaugurated on 20th Jan 2021.
H&M confirmed they were not coming back to the UK shortly after, on 19th Feb 2021.
They obviously feel safe under Biden for whatever reason. It has never been revealed what kind of visa Harry is on. But Biden and Charles know. I rather imagine Sunak and Starmer and been briefed. As pp have said, Harry seems to be untouchable, no matter what drugs revelations etc he makes money out of, or where he goes and who he associates with, including in Nigeria. My guess is some sort of diplomatic/A1/HoS one, which will be due to expire in the next year and so, regarding his residency status, Harry has to shit or get off the pot.
If he can cease to be an Internationally Protected Person (IPP), as he did on the 31st March 2020, then he can cease to have a certain type of visa when it finally runs out.