I don't think many people use it these days.

That's probably going to be hard to estimate as it is enabled in scripts via the shebang line and scripts are less likely to be open source and published than other code. However, I use taint mode as much now as I did 10 years ago and find it a very useful line of defence. Proper handling of tainted data is a skill in itself and maybe it isn't taught so much now. That would be a shame.