Let's say I have video where I need to cut out background. I can convert video to many frames and then edit each frame. But it's a lot of job, so would be convenient to have opportunity to copy shape of selection and slightly edit it, so it would fit next frame. It seems there no easy way to do it in GIMP. It should be at least cheap. Preferably for Linux or Android. But Windows is acceptable.
The free video editor Kdenlive for Linux (and Windows) appears to have the ability to accomplish your goal.
A tutorial of sorts, perhaps more a guideline-type entry suggests to use GIMP to create a mask for your video project. You would import a frame of the video to GIMP, select and delete/adjust transparency of that frame and save it in the appropriate format for Kdenlive (.pgm) which is then used to mask the video.
Obviously, with video, there is likely to be movement in the video that would require targeted masks. Perhaps you would find that you can "block mask" sections of the video to reduce the workload.