Oh sure, newer versions of iOS used to slow things down, but there are two separate issues.
1. Do increasingly heavy OS releases slow older machines (yes. Though ios12 has reversed this trend.)
2. Does iOS deliberately throttle older machines.
The latter is the particular issue. It relates to 2 or 3 iPhone models with only when they have dodgy batteries, so I don’t think the accusation of generally nerfing old phones really holds.
I do agree, however that Apple absolutely should have made it clear what was happening. Very poor communication, but not, I think malicious