I mean, it also depends on how reliable the engine is in regards to relighting, also wether or not it has a way to settle the fuel before firing. Might be easier just to have a few small solid rocket engines added to the stage to give it enough oomph to get it to aero capture in a reasonable amount of time. Admittedly this would probably require extensive testing to ensure that load of the deorbit motor is spread in a way that won’t just shred the stage.
Or maybe they just use some engines on the satellite to get it in to it’s final orbit and release it from the upper stage while the upper stage is in an orbit that will decay fast enough to be considered acceptable.
This last one is the one that makes the most sense to me. It requires the least extra engineering on delivery vehicles and could work for most larger satellites that may need to be able to maneuver on orbit anyway. Vehicles delivering cube sats might need a de-orbiting booster on their stage though. In that case it probably makes most sense to build the final stage with a little extra dv and just burn retrograde after delivery.
There will have to be some cost trade-offs for the big GEO sats. What is the extra fuel worth to the satellite owner? Or expedited delivery to the target orbit? Is it worth the cost of sending a separate little deorbit tug up for the 2nd stage? Or a separate refuelling mission somewhere down the line?
