Correct me if I am wrong, but as far as I can see the following is not yet possible: create a selection on a frame, optionale change this selection on the following frames, and then heal this selection (or those selections) on all frames. If so, count this as a feature wish.

Use case 1: I received an animation from a supplier to promote their product on our social media and I want to add our branding to it. For this I had to hide an element in their animation to make space for our own logo.

Use case 2: remove a disturbing element from your video to make it better. For instance, if you create wine tasting videos and made a good shot but afterwards you see you spilled red wine on your white shirt, you can try to hide this stain in the video instead of redoing the whole video.

My sollution for now was to export multiple frames as image, open each of them in GIMP, use the Resynth plugin to cut out the specific element, make everything transparant except for the part that I want hide, export these images and import them in Kdenlive, put them over the video on the right places. Obviously, this is not as userfriendly and productive as it could be.