On my hybrid domain sometimes changing msexchhidefromaddresslists to True in ADUC hides the user from the GAL, and other times it doesn’t.
Turns out the mailNickName attribute needs to be populated with something for msExchHhideFromFddressLists to be honoured. I just populated it with firstinitial.surname.
Waited for a sync and bingo bongo they were now hidden from the GAL.
I do have an on prem exchange but I never use it. I only have it because MS says I have to. I never use the admin console as I find for the few tasks I need to do (hiding users from the GAL being one of them) I can use the the attribute editor in ADUC just fine.