Usually when you see a new docker_manager release it’s because a new Discourse release was announced in your admin notifications, so you would most probably do 1. upgrade docker_manager, then 2. upgrade all, 80 to 90% of the time.
When you want to check for a single plugin you scroll down anyway. I’m simply thinking about wasted resources. After some years of upgrades, across thousands of instances, that would make a lot of saved resources.
I suppose a middle ground would be to kill the check requests when the docker_manager upgrade button is clicked. But I would expect that simply not checking when this plugin is out-of-date would be the most sensible – I do not have stats, maybe you do.