Minecraft Java Edition – Do Mods Need to Be Removed to Play Multiplayer

My favorite kind of answer to my favorite kind of question:

It depends

Some mods, like mods regarding high-res textures, are single-player mods that do not affect the communication with the server, and can therefore be used irrespective of server configuration.

Other mods, like aircraft mods, require server support in order to work.

In general, the description of any given mod will list if it's compatible with vanilla servers, if it only takes effect in SSP, or if it requires a modded server.

Edit: In response to comments; one piece of clarification:

Some mods function somewhat more like exploits based on the fact that the server allows a certain amount of client-side processing of things like falling. Mods that function by misrepresenting the effects of gravity, for instance, can function without server support, at least until the communication protocol is changed to handle all movement server-side, assuming that ever happens.

Get and use MagicLauncher instead of the vanilla Minecraft launcher, and use its "profile" manager to set up one profile with mods for single-player and another profile without mods for multiplayer.

Because it loads mods dynamically, you don't have to alter the minecraft.jar, leaving you a perfectly-vanilla game for multiplayer. In your single-player profile's setup just add the mods to the lists of internal and external mods, and the launcher will add them to Minecraft only when you start the game with the single-player profile. When you want to play online, quit and restart to login with the vanilla, multiplayer profile.