Yes, that's a myth. Dogs are spayed now when they're IN heat, pregnant, if they've gone through heat, etc. We adopted our female when she was 4-5 years old, the rescue spayed her before we got her, she had obviously gone through a heat cycle. Personally I've never heard that myth, but if it's going around, then yes it's absolutely a myth as vets will spay dogs if they're in heat or pregnant (extra cost typically), so anything goes nowadays.
Personally I spay/neuter at around 6 months, but there are pros and cons for early and late so, do some research and then you will have to decide what is best for your dog and your individual situation.
Best of luck!