What do your readers recomend for old stains in carpet?

Steam clean

browse through others posts. https://www.hometalk.com/search/posts?filter=carpets

You can try spraying undiluted hydrogen peroxide on the stains. Let it sit for a few minutes and blot with an old tee shirt or other soft cloth. (Test first on a hidden area of the carpet, but I’ve never had an issue.)

Also, steam cleaning.

Resolve is good at getting that traffic pattern grime out with their soft carpet brush which I put on a long wooden pole, then I like to shampoo the whole rug.

I really like Folex and it is cheap. My 2nd choice is Resolve carpet spray. Just don't use shaving cream. It does a great job "on the spot", pardon the pun, but it attracts more dirt much faster, and builds up, speaking from experience.

https://www.homedepot.com/p/Folex-128-oz-Instant-Spot-Remover-FSR128/202820335