It was just announced in https://groups.google.com/forum/#!topic ... E8Yw6POxEE that they intend to drop all support for legacy add-ons within the version 72 time frame (by end of this year). Version 68 removed a lot of support for legacy add-ons but it was still possible to use one if it was appropriately modified. "He is talking about dropping support for the "legacy" key in your manifest.json. Which means only true WebExtensions will be supported."
They are planning on integrating Lightning into Thunderbird, rather than have it be a bundled add-on, as part of this effort.
"Dropping support for non-MailExtension add-ons is also needed for addons.thunderbird.net. Supporting old-style add-ons would require a significant investment in the back-end there, since the Django version of the back-end would reach EOL and have to go through a painful and expensive upgrade" seems to imply that legacy add-ons would not be available on thunderbird.net anymore for anybody who choose to stick with an older version of Thunderbird. That seems like a very bad idea, if true. It also sends a terrible message to all of the add-on authors that have been updating their legacy add-ons to keep them working despite all of the changes in the environment.
Ryan Sipes is the community manager. His email address is listed in https://wiki.mozilla.org/Thunderbird/Core_Team
