You are referring to the iPhone 6 and iPhone 7 when Apple discovered a battery issue and the fact that batteries at a certain health would suddenly fail. To avoid this (I mean it is a phone you might need to call someone) if the battery was reporting the issue they slowed down the processes to avoid sudden failure. The mistake they made was they didn't tell people they were doing this. They updated iOS to show battery health and if your battery had an issue and gave the option of slowing the processor or just risking the phone shutting down. This type of action had been used for the past 2 decades on laptops. Apple admitted they did this, told everyone why, settled their legal issue, and provided options as I previously mentioned. You can actually see if the iOS is offering to slow the battery.
On a much older phone, like the 8, with slower (or less) RAM an update could result in a minor slowdown. Same for the Google OS.
I've seen no slowdown on iOS 16.