This is exactly my use-case for a fossil repo with the built-in forum, with the default provided sorting. Fleeting notes get migrated as needed to Wiki pages (capability for which is also built-in).

Demonstration of default settings: https://fossil-scm.org/forum/forum ("most recent threads" is equivalent to "most recently updated thread")

Docs: https://fossil-scm.org/home/doc/trunk/www/forum.wiki