I know the Stack Overflow developer survey can be pretty good to understand what's happening now, but I'm more curious about long term projections.
Is this information available anywhere?
The SO developer survey and others [0] [1] [2] have been around long enough to notice trends, but projection - whether in economics, sports, or the weather - is of course about much more than trends.
Personally I would seek out the advice of as many trusted greybeards as I had access to; experience counts more than math and statistics. Also, if your research is for the purpose of guiding current decisions (you didn't say this explicitly but I picked up an implicit need) be aware of black swan events [3].
[0] https://www.jetbrains.com/lp/devecosystem-2022/
[1] https://redmonk.com/sogrady/2023/05/16/language-rankings-1-2...
[2] https://www.codingdojo.com/blog/top-programming-languages