That rule still exists. It didn't apply to Johansson because his Luxembourg caps were before his 21st birthday.
Clarke-Salter was capped for England Under 21 after his 21st birthday, but he gets off on a technicality. Following pressure from a player/association (can't remember which one, it might have been a player that wanted to play for Morocco but had underage caps for Spain), that rule was amended so that it only applies to underage caps obtained after September 2020, when the new rules came in.
The justification was that it would be unfair to penalise someone who took underage caps before that date, obviously not knowing that the rules were going to change.
So, in Clarke Salter's case he was capped for England Under 21 after his 21st birthday, but that was in 2019. So he should be fine to switch as long as he has a grandparent born in Ireland. But if someone younger tried to do the same thing and won those England caps after September 2020, they'd be ineligible.
Edit - the test case player was Munir El-Haddadi.
https://wap.business-standard.com/ar...0501668_1.html