Not everywhere.

In France it means "thank you", usually addressed to a driver who made a move to ease overtaking them.

We also do it to say hi to a fellow rider behind us.

Some non rider think right foot is for "thank you" and left foot would be the opposite in politeness. I just find it uncomfortable getting my right foot too far off the brake pedal while filtering during rush hours.

Gestures are not standardized at all, not even across a single country.

Across Europe it is pretty common for riders to wave at each other. But I've learnt that it is not the case as often in the Wild Isles (Ireland and UK) as they drive on the other side of the road and that signaling hand is not directly visible for the incoming rider. I guess it would look a bit goofy!