Leicester City fan here, in peace as usual.
There weren't any Leicester City (or Villa) fans there to boo him last season because of COVID, but as I recall, he got some (fairly good natured) stick the previous season when we played Villa four times, including losing on aggregate in the Milk Cup and putting four past them home and away in the League. A lot of us think he's a cheating little **** who falls over a bit to easily, although to be fair, a lot of us would also love him to be playing for Leicester City.
Incidentally, I don't think booing him is anything to do with the Euros and it's not just a Midlands thing - Grealish will get booed all over the country. But
@Keeper!'s comment is the most sensible - time to worry when people start ignoring him.
And I enjoyed yesterday, so there :)