Doesn't actually refer to `OP_RETURN`, actually. Still browsing to see what the first value was when that variable was first established. This is very old source code by now. Sourcegraph not very useful in way back history.
Discussion
Flavian:
"The maximum size for OP_RETURN outputs used to be 80 bytes"
"Also, that value is now configurable through the datacarriersize option, so miners who want to stay on 40 bytes (or any other value) can easily do so."