Are 1934 US 20 dollar bills in circulation?

It's extremely doubtful. The 1934 series ended in the late 1940s so the chances that a bill would survive that long in general circulation are extremely low. Banks remove worn currency and return it to the Treasury for destruction, and it's also very possible that any old bill could be snapped up as a keepsake, collectible, or curiOSity before it's removed by a bank.

About the only way you'd find one in circulation would be if someone had old bills saved away somewhere (a drawer, shoebox, etc.) and spent them at face value, AND you were next in line to receive them in change. That scenario can and does happen.

It's worth noting that except for gold certificates, the Treasury has never withdrawn or demonetized any standard-issue paper money. No matter how old a bill is it's still technically legal tender at face value. Of course spending a collectible bill at face value isn't really a smart financial move!