Did they move? When I installed Thunderbird on my Windows 10 machine recently, I noted that Maildir was available as an option, and switched to it. But it seemed like mbox was still the default, and certain behavior seems to be glitchy on Maildir. (I get frequent notifications about compaction of mailboxes, despite the fact that Maildir cannot be, and does not need to be, compacted.)
you're right, it's a work-in-progress. It was introduced in v60 in 2018 as experimental feature and enabling it in v78 still holds a warning to it.
I didn't think maildir worked on windows at all due to the colons in the filename (or does TB use a different character for that on windows?)
Currently using it with maildir on Windows 10.
Oh that probably fixed most of the problems then; mbox is a terrible, no good, very-bad format.