I have a friend whose phone slowed down quite a bit, he did the usual
stuff (clear cache, remove unneeded apps including stuff running in the
background) but it's still frustratingly slow. It's a Samsung and it's 4
years old and he uses it a lot.

I read about write cycle limits per memory cell he

"Note that TLC is a type of NAND memory pioneered by Samsung. It's the
cheapest to produce but has the worst durability: 4,000 write cycles per
cell versus 10,000 in the more standard MLC type. This might be why
Samsung devices have a reputation for slowing down more than non-Samsung
devices."
https://www.makeuseof.com/tag/phones-slow-usage-years/

I was wondering if anyone knows of an app that shows the write count for
memory in an Android phone.