Particles are not handled server side They are packets wich are one time sent and then the server looses all the control about them! So they are already gone after the packet was pushed through your internet conmection
They are FloatingTextParticle! If you do not store your reference to it server side, it will linger in the player session forever. Nothing to do with disabling plugins. Exactly for this reason, you should despawn them upon plugin disabling if you are a responsible developer.