“Furthermore, in the entire 20 year lifetime of the Thunderbird project, no one has added support for a new mail protocol before.”
Thunderbird’s mail architecture predates Thunderbird itself and dates back to Netscape Communicator. From 2012 to 2017 nobody was paid to maintain it. Native Exchange support lands in the July 2024 ESR, email only, with calendar and contacts later. Twenty years of decaying C++ and the fix is a new language bolted on through a Cargo workspace hack.