I'm a maintainer of the projectM visualizer library. We've been working on making projectM better in many ways over the past two years or so, and finally released a new major version (4.0) back in April '23. This new version fixed a few bugs and improved the rendering quality, but was mainly focused on rewriting the build system (now only using CMake) and creating a proper API. libprojectM now supports building as a shared library on all platforms, and has a stable and clean C API instead of requiring direct access to the internal C++ code, which was a big issue for devs in all previous releases.
Clementine used a forked copy of a very old projectM version, which was never updated and had numerous bugs and performance issues, so removing it from Strawberry was surely a good thing. Yet I think many users would like to have a visualizer again, and with the upcoming 4.1 release projectM will be (mostly) compatible with the latest Milkdrop release, support previously unimplemented preset features like loops and megabuf.
The API won't change between 4.0 and 4.1, so if anyone has time to integrate it into Strawberry, you may start right away. If there are any questions, head over to our Discord or post in our GitHub discussion board, I'll be notified there. I try to check this forum from time to time, but expect longer delays here.