No. Children with disabilities don't lose their disabilities when they turn 18.
I think the decreased number of people linked to
DAS is to try to target the unofficial tour guides who were using DAS to offer shorter wait for their clients. Disney has been cracking down on unofficial tour guides for a while and the ones using DAS for a 'tour tool' are especially reprehensible.
I think the number is also aimed at 'Influencers', especially some liverstreamers who brought in a large rotating group of group of guests.
Then, there are also people who brought in large groups of extended family members - aunts, uncles, cousins - I've seen people talk about as many as 16. Also the guests who wrote they had 4 members of their family registered for DAS and each person had 5 linked to their DAS (so total of 24 people)