I've got an SLX derailleur on my gravel bike with a 1x setup.

It outperforms the late 90s XTR that I had when it was the best thing. It's smoother, quieter, shifts better, and holds it's adjustments better. The clutch is a huge improvement in keeping the chain on. It'll shift a range that was just impossible 20 years ago.

That said, I had a 1994 Fuji road bike that shifted just as well as a new bike. It had 600 Tricolor, that's about ultegra these days. The old stuff is limited to tighter range cassettes and fewer gears but it works fine.

I would be careful shopping for an old bike though. A lot won't fit tires bigger than 23.