Support for legacy extensions was removed from the Thunderbird Beta version 74, released in February 2020.
The next major release version of Thunderbird, version 78, will be out around June 2020. Until then, code wise many things are going to change. For instance, what is left of XUL will be gradually going away. Thunderbird's XUL documents have already been converted to XHTML, and will have less and less of a XUL flavor until the conversion from XUL to HTML is complete.
Authors and maintainers of legacy add-ons (including MailExtensions with legacy support), have two routes forward:
Convert your add-on to a MailExtension. If the API you need doesn't exist yet, tell us about it.
Convert your add-on to a Web Extension Experiment. Most add-ons should be able to be converted to an experiment with a reasonable effort.
The recommended path forward is to convert it to a MailExtension. That will make sure the add-on works without significant changes over many years. If you go with option 2, you'll have to maintain a lot more code yourself and breakages can and will be bad unless you keep up really close.
MailExtension Experiments should be seen as such, experiments with the goal of getting the API they need into Thunderbird core. Please work with us on getting the needed pieces in as a supported API. Initially we'll be allowing experiments to be exposed to the general public, but over time (years) Thunderbird will gravitate towards not having the experiments available to the general public, the same way it works for Firefox.