A similar thing happened with BCPT when it turned into a REIT. Before it changed to a REIT you could buy/sell in an ISA. Then BCPT changed to a REIT and you could only buy/sell if you had existing holdings in BCPT. More recently it’s changed again so you can only sell BCPT holdings in an ISA.
I would have thought it makes send to let existing investors buy/sell given Freetrade already has to do extra work to get the 20% credit on REIT dividends, which is the reason why REITs are not included in an ISA ( see this answer: UK REITs in ISAs 🇬🇧 - #15 by Gemhappe ).
Anyway, that’s a very long way to say I’d prefer if investors could buy/sell on open positions that were opened before Plus launched ( to avoid anyone signing up for a month buying one share in a bunch of Plus only stocks and gaming the system ).