I know you lose avios after 3 years of inactivity, but when does BA delete you BA account and membership ?

My daughters account was not showing up when I tried to add it to my household account and BA are telling her that it as deleted due to not logging-on and lack of activity. I knew she would lose her avios after 3 years, but surprised her account and associated life tier points were deleted. She last flew 4 years last Sept, as starting a family and pandmic has effected her.
Is this correct that accounts get deleted after a set period, and if so how long ? Or could it have been a tech issue?