Thunderbird version 78.14.0 shows defect and also the latest version 91.3.2
Account Settings > Disc Space settings
OR Imap Synchronisation & Space > Disk Space
Default is 'Don't delete any messages'
If you want to select any other option, it is impossible to set the number of messages or the number of days.
You have to select the checkbox to enable access to set number of messages /days.
However, a pop up window immediately appears:
'This setting will permanently delete old messages from the remote server AND your local storage.
Are you sure you want to proceed. Ok Cancel buttons.
This pop up window has taken focus, so you cannot select the number of messages/days.
If you cancel the pop up message, then it auto deselects the option and selects the 'Don't delete any messages' option.
This means if you do not want to use whatever numbers are showing, you cannot use that function without risking deleting emails you do not want to delete.
User in Support Forum also mentions this problem.