https://madnight.github.io/githut/#/pushes/2023/4
Unfortunately it doesn't have new projects, but it does seem like C++ peaked a couple of years ago and is starting to trend down. "Plummeting" is clearly an exaggeration though.
(WTF is there a meteoric Java growth is yet to be explained.)
Either that or the institutional memory of Java EE has faded, and Spring Boot is here to traumatise a new generation of developers with the magic of write-only COME FROM code.
Those things you cited do no cause single-month rapid growth. What causes it is publicity, or some large ecosystem adopting it by fiat.
But yeah a 0.234% yoy-decrease isn't "plummeting". If so, then things like typescript or python are even steeper in their, and java being the next big thing showing massive growth compared. And rust itself is decreasing according to those stats, so I guess it's declining too.
In most metrics there it's steadily rising even, though the metrics are cross-correlated with "GitHub adoption".