It’s difficult to say for sure what the future will hold for Thunderbird extensions. A lot of work has been done to ensure that legacy extension (overlay extension and bootstrap extensions) will work in Thunderbird 68. Beyond that, we just don’t know.
Overlay extensions are problematic because so much of what they depended on no longer exists. Bootstrapped extensions are less of a problem but are still considered at-risk.
There are two types of changes which might be required to make your legacy extensions compatible with Thunderbird 68:
All extensions need to change to the new manifest format.
Overlay extensions need to engage the new overlay loader. The overlay loader is a Thunderbird component that takes XUL code as written in an overlay extension and applies it to the UI. In Thunderbird 60, this was a part of the core UI libary, but it was removed. We have built a new overlay loader to replace as much of the removed code as possible.
All extensions need to be updated to reflect changes in Thunderbird core, like renamed/replaced API calls, removed support for some XUL elements (need to use HTML elements now) and much more.
Remember: Thunderbird is based on the Firefox code, and they have been changing things all over the place.