I thought I'd add that I tested these changes with my skeleton script that reproduced the error for us and it seems to be working. I also can't make these changes to our main Jenkins instance, but I used my docker setup that I have for reproducing errors.
Previously, I had narrowed down the cause for us to be the number of stages with "when" conditionals. When we get somewhere between 30 and 35 stages with "when" expressions, the error shows up, regardless of any other code in the pipeline (I was able to reproduce with a blank pipeline library with just echo lines).
I installed the plugins and activated SCRIPT_SPLITTING_TRANSFORMATION, and now I've been able to run the same script with 60 stages without hitting the error. I might be able to go higher, but our use case is far from hitting that many stages.
I do want to say thanks for keeping this issue active. We've been running a workaround script for a while now but I've been keeping my eye on progress on this issue, and it looks promising so far. I'm anxious to get back to a pure declarative implementation.