It’s fixed now! Old and pinned threads now show up all the time
For communities that gather or archive content, it’s not great for old threads to disappear, unless a different sort mode is selected. Active sort (the default) hides old threads and it’s not immediately obvious to the potential visitor that there is more to be found by changing the sort mode.
This also affects pinned threads, that are simply invisible, unless somebody recently participated in them.
Have also been wondering this! Would be lovely to see as an option.