The space station's orbit has been adjusted continuously over its lifetime initially by attaching a shuttle to it and doing a burn of the shuttle's engines and later doing the same with progress modules.

My bet is the original expectation of the designers was to deorbit by attaching centaurs (or whatever) to the existing docking ports and rotate the beast to the right attitude for a deorbit burn.

NASA has more recently said they want the reentry to be as steep as possible to minimize the size of the debris field, and is using that to justify the development of a new specialized deorbit vehicle. No doubt SpaceX will declare that Starship is the proper vehicle for this, and then will plow the $800 million into the Starship program. The money they got for Artemus is already long gone and Starship has failed to demonstrate key components of the Artemus plan. Dear Moon has been cancelled so NASA and Artemus are the only customers they have left. NASA knows that without a cash injection Artemus is at risk.

I was kind of hoping for Impulse Space, but they're probably too unproven.

Maybe the countries who put it up there should have had a plan for taking it down? Or at least pay for it?

Their failure is a huge opportunity for the usual grifters.