When we used thunderbird and archived emails because changing computer or changing job/place we could archive and re-import old email in .msf and .sdb
This is still necessary today. It is not possible to import the .msf and .sdb as before to consult and old email that contains a contact or a receipt or an agreement.
We must be able to import and re-read .msf and .sdb folder from our email history.
I would suggest you make your support request on the support site. Be informed. SDB folders are a signifier to Thunderbird that a sub folder exists with the name before the extension. MSF files are a generated index and contain no emails. Neither have ever been of any use for backing up or restoring mail as such. The mail is stored in a file with no file extension with the same name as the MSF file.