I looked everywhere and this seems to be the best place to post my dissatisfaction. I do not like the native support for OpenPGP now included in Thunderbird nor the fact Enigmail chose to retire the add-on. Also, using Linux and its package manager I had no way to be warned about the changes during the Thunderbird update (providing the update wizard did warn about the relevant changes!). The only viable option for me right now is to try to downgrade the Thunderbird (and the user profile!) and wait and see how the situation unfolds.

This is extremely disappointing.