> Do you need to prohibit Java 20 to teach inheritance?

Java20 did not introduced inheritance, nor did it introduced any OO concept. Java20 adds no value to those learning OO. If you're writing java code but are not using any Java20 feature, you are not writing Java20 code.

> Working with an outdated version (...)

It is not outdated. You are not using any feature. You're just succumbing to the misplaced belief that new means more value. It doesn't, specially when the only thing that you get is more complexity.

> Sure, but that doesn't mean knowledge of newer features (...)

Repeat after me: Java20 adds no OO feature. You do not have to use Java20 to learn OO. OO courses are not java courses. Do you understand this?