That does not apply to royal family titles:
(Only to 5 ranks of peerage: https://en.wikipedia.org/wiki/Peerages_in_the_United_Kingdom#Privilege_of_peerage)
“His/Her Royal Highness” is a Style, not a title.
Effectively they do not have the HRH anymore. But their children are eligible, now that Charles is King.
Harry is also about to be deprived of being a Counselor of State since he does not live in the UK and is now fifth in line for the throne.