That has never been replicated?
He would be incorrect saying that it worked like that prior to 4.19.
Also setting it on the server dynamically isn’t going to work correctly because the saved movements coming in from the client after that is set will be run at a different speed on the server, causing a momentary hitch when the speed changes.
That is unless it is run on begin play like he had originally.
This is why the characters alter movement speed when crouching and toggle speed during crouch, the crouching flag is passed up from the client to the server in the SavedMovements replication.