Let's suppose you are playing the simplest of beats, let's say you just play a note/tone at 120bpm. There's 0.5 seconds between your playing.

To do a linear speedup, you would gradually reduce the time between notes, maybe like this: 0.5, 0.45, 0.40, 0.35, 0.30 and so on

Problem is, it turns out that humans (even the musical among us) are not very good at all at measuring absolute time. What we are good at is measuring relative time, and so what actually happens is that we decrease (or increase) the "time per note" by a constant *factor*. Let's say the factor is 0.1. So now, the first note of our nominal accelarando is 0.45 long, but the next one is 0.405 then 0.3645 then 0.3285 and so forth. This forms a natural exponential progression.

Put a different way, human performers do not speed up or slow down by a constant amount per beat of an accelerando or decelerando, they speed up or slow down by a constant ratio.

This behavior has been noted in several research papers on human musical performance, and it is very, very difficult for most humans to do anything else.

Alas, I have no specific info on this. x42/robin gareus might know more. Not many plugins care about the tempo map. The ones that do have a lot of potential for screwing up if they come with some baked in idea of "this is what accelerando sounds like".