@team_creativebuffer I believe by Plugin close, you mean close of the panel(window created for plugin),
before jumping into more details, this is provided on UXP which also requires some implementation from the Indesign, which is not done yet, you can expect this to work in the later releases
Coming into more detail, Developers often confuse the terms plugin and panel in UXP,
What you are referring to above is a Panel and not Plugin,
Plugin creation and destruction are managed by the application i.e. Indesign in this case.
Invoking and closing of the entrypoints like Panel/Commands are based on the user actions
Plugin is created during the App launch and destroyed during the App Quit. UXP does have lifecycle events for these which can be referred here
Panel opening/invocation & closing/destroy are based on the User interaction like opening the panel from the Plugins Panel. Opening/Invocation of Panel entrypoint would result in Panel lifecycle events Create and Show more details refer here
Similarly, Panel closing would result in Hide/Destroy events.
Plugin Events(Create/Destroy) should be working as there is no implementation required from the host, but as mentioned these would come during the launch/Quit of the App respectively.
Panel events would be more on the Panel, like open and close, etc, but these need some work on Indesign and which is not done yet, so these wouldn’t work yet.