Only if you're using a ridiculously outdated copy:
changed
Add-on support: Add-ons are only supported if add-on authors have adapted them
changed
Dictionary support: Only WebExtension dictionaries are supported now. Both addons.mozilla.org and addons.thunderbird.net now provide WebExtension dictionaries.
changed
Theme support: Only WebExtension themes are supported now. Both addons.mozilla.org and addons.thunderbird.net now provide WebExtension themes.
Literally here's a doc explaining how XUL has changed as of Thunderbird 68, the most recent version, released about a month and a half ago. Yes, some elements have been removed, but others have been modified and still exist.
And that's in the add-on documentation, not even just internal development docs.
Also, describing information changed in the most recent stable release, a month and a half ago, hardly qualifies any older as "ridiculously outdated ".
