OK - I'll confess that was a bit of a clickbaity subject line. 
I know many will shrug their shoulders at this one, but when MNHQ first proposed that they would remove the 90-day deletion rule from Chat, there was some objection to its sudden introduction, so they eventually agreed to a delay giving notice that they would enforce it from 9 November. However, threads from 11 August are still there, and that's over 140 days ago, so they should have gone by now!
8th November = 8/11/2018
11th August = 11/8/2018
So I suspect some American-format shenanigans: cock-up rather than conspiracy. 
www.mumsnet.com/Talk/site_stuff/3338581-MNHQ-here-removing-the-90-day-limit-on-Chat-threads?pg=7