Hide gamemode change announcements

Solution 1:

You can create a server resource pack that the clients will by default automatically download. In it, you can have a lot of language files, one for each language, all the same, containing only the line to replace the gamemode update message with nothing (or a space, whatever works).

With this, players will see an empty line in chat. So if you also have other things in chat, they will still get shifted up.

Using this method, the message is by default deactivated for everyone, but players who know what's going on can enable it if they want to.

Solution 2:

type /gamerule logAdminCommands false, this should do the trick so when you type commands in it shouldn't display it to the player in question.