https://forums.mangadex.org/threads/kyomu-nee-chan-wa-kamatte-hoshii-ch-1.2601173/#post-29525207
There's still a chance that there aren't removed image that I used it on them but couldn't find them, if that's not the case then maybe all the images I used it on them got removed, and only some of the image that I didn't use it on them got removed. This is an old one but maybe it's an exception because I used PicsArt to edit nezuko after using "remove image background" on the woman
https://i.postimg.cc/fTj07tB4/image-removebg-previewdcccc.png&hash=88a91ca87f41402912d757da921b94e1 In any case, all the images failing to load you linked are unavailable on postimg's end. You can check by grabbing the image link (in md forums) and url decoding the image query parameter to get the source URL...
So yeah, you'll have to figure out why your images become unavailable or use a different service, there's nothing on MD's end to be done about it.