Most professional stadiums display the pitch speed, albeit usually on one of the auxiliary boards rather than the main video board.

Thanks to enhanced pitch tracking in the last few years, they can now display even more information. The Pittsburgh Pirates' first- and third-base ribbon boards show pitch speed, vertical and horizontal break, and IIRC even the name of the pitch (bucketed based on the speed and break characteristics). It's a really neat addition to have in real time.

When the Cubs signed Aroldis Chapman I went to the first game where he came out of the bullpen and I will never forget the entire crowd looking at the scoreboard and reacting every time he hit 100 miles an hour. Now every team has somebody that hits 100. It blows my mind people can throw a baseball faster than I drive on the interstate.

Given than scoreboards were updated by hand, and could be seen from the plate, one wonders if the reported pitch speed was ever altered to perhaps confuse a batter. The one person in the stadium not able to see the scoreboard is the pitcher, the one person with input on pitch speed.

Did you expect time travel was developed, while no one was looking?