Hi everyone!
As of December 2019, we disabled permissions to edit the subject line of threads that are authored by another user. This change is applied to all user tiers on Discussions.
And some of your observations are right — authors of threads are given a period of 2 months (86400 minutes) after posting to edit their own post, after which the author will no longer be able to make changes. This has been the default setting on Discussions (since the beginning of time), and rest assured no changes have been made to the permissions for editing the subject line of the threads that you authored.
I hope this adds some clarity for everyone!
Take care and stay healthy!