The latest on this was from June’s Q&A: June 2024 Developer Stream Transcript
[Jorg] For sure. So first off, Backwards compatibility is something we said we’re doing. So we are, you know, we have some people like Gaya that work with us basically as a first party developer and we’re using them to test the tools, make sure that everything works. It’s hunky dory and they give us feedback and not, I would say not everything was perfect. So we’re still, the team is still working on making sure that there’s no impact because the whole point of backwards compatibility is that you don’t actually have to do anything. It’ll just move over. So I think it’s massive progress was made. But to answer the actual question, will the SDK be available prior to launch? For sure. It’s not quite ready yet.
Bottom line, they are striving for minimal to zero changes in existing add-ons to work in MSFS 2024. To take advantage of any new features and capabilities however the 3rd party devs would need to do extra work.