This is very interesting and the first time I’ve heard anyone discuss this. Do you have data / benchmarks you could point to? It would certainly address a lot of “planned obsolescence” criticisms by making it more plain the cause of the slowdown.
Unfortunately this mostly comes from my professional projects as Android developer and large-scale profiling data. It was confirmed by many devices I got to debug and "fix" when slowed down, but that's more anectodal.
I don't really have links to actual research on phones though - although eMMC degradation is a pretty researched topic itself.
The main reason for this is hardware though - there's always plenty of conspiracy theories around that, but the biggest issues we've seen across Android ecosystem are due to flash.
Cheap eMMC flash with lowend controllers degrades pretty quickly (within 3-4 years) and you suddenly start seeing like 20-3000ms read times of a single settings file. Since a lot of apps just read those files on UI thread, this results in a lot of stuttering because developers expect < 16ms response times for those reads. Writes get even worse - it's not rare to see 500ms+ times for writes to configuration files of < 1KB in size, especially if there are multiple parallel writes queued (common in browsers when they cache webpages for example).
Apple and higher end Android devices tend to use better SSD quality flash which tends to last years longer than eMMC one and thus the devices don't slow down that much.