In 1.15.2 snapshots, pre-release, it was added a new feature that changes the title depending on the type of game you are playing. If you are playing Singleplayer world, the title window will change to "Minecraft 1.15.2 Pre-release 2 - Singleplayer", this also works on multiplayer worlds, for LAN worlds, it will say "Multiplayer (LAN)" instead of Singleplayer at the end of the title.

Therefore it also changes the title of the window for Multiplayer servers, "Minecraft 1.15.2 Pre-release 2 - Multiplayer (3rd-party)", this makes the game seem modded, which is false. It's just using a vanilla server hosted on a machine.
I believe it's much nicer to have something as "Multiplayer (Online)" or simply just "Multiplayer" instead of the "Multiplayer (3rd-party)", and instead use the 3rd-party part for modded Minecraft.

This doesn't affect the gameplay but does the title and meaning of the mode playing.

(Image below is just a resource pack, the only 1.15.2 server I have at the moment due to multiplayer testing. Having a resource pack does not affect the title, already tested.)

Note: If this is a duplicate of an already existing report, I am sorry, but I didn't find any of the reports when I was searching for it. If needed, feel free to close the report.