Most? Absolute nonsense. The Nexus 7 have a small issue that was resolved when Google added TRIM support for Android. That was years ago.

I have a long history with almost every Android device. The device doesn't get slower, the battery life just gets terrible.

My Nexus 6P's performance has degraded considerably over the past couple of years. It's actually quite laggy now. A few of my friends report the same with their 6Ps. To be fair I haven't tried factory resetting it in a while, although I shouldn't need to do that in the first place..

The 2012 Nexus 7 is an outlier because it has especially bad flash controller - by 2015 most of the devices were unusable due to degraded flash. TRIM helped very little.

Other devices suffer from similar effects - not as bad, but most devices certanly lose quite a bit of I/O performance (especially writes) over time which makes OS run noticably slower.

Flash degradation literally only affects writes. Android has had TRIM since 2013. Writes in no universe make the OS run "noticeably slower" (or lead to "lag"), even if your claim was proven.

There are literally billions of Android devices. Benchmarking file system performance is trivial. Ergo, where are the benchmarks supporting your broad claims? This should be incredibly easy to demonstrate. We do know that it happened pre TRIM because people had factory new and after use benchmarks, and could show the decline.

I've owned two low cost Moto G phones and never had this issue.

Is this true for flagship Android devices too?

Depends really, a lot of them nowdays use better flash controllers and technology (e.g. UFS) but only time can tell how they cope with aging. Pixels seem to be doing quite well.