[peeters]

Most would consider it to be vanilla, when "vanilla" is used as a descriptor of the gameplay (i.e., as opposed to "modded"). There are some very minor differences, but almost anyone advertising a vanilla Minecraft multiplayer server is going to be using Paper or one of its predecessors/competitors. The released server.jar just really isn't optimized for more than one player.

Some things that you can tweak for performance reasons also affect gameplay. For example, you can tweak the radius at which items group together into stacks of entities. Enabling this can make certain farm builds not work, e.g. a piglin bartering farm might have the gold bar destined for one piglin merge with the bar destined for a neighbouring piglin, when in pure Vanilla settings that might not happen. Similarly, you can have XP orbs merge into single orbs with higher XP values. But this drastically changes how long it takes to use an efficient XP farm, where normally you would be waiting for minutes for your player to receive the XP one orb at time.