One other example of the "bitmask" type of platformer (although you probably wouldn't call it a "platformer", at least not a "jump'n'run" game, as far as the genre is concerned) is Lemmings. The levels of the original Lemmings games are definitely not saved as bitmaps (at 120 levels, that would have far exceeded the capacity of an Amiga floppy), and they're not tile-based either. AFAIK the levels (https://www.mobygames.com/game/amiga/lemmings/screenshots/ga...) were created by repeating certain graphical elements (e.g. the columns in the screenshot are vertical repetitions of one element, the capitals of the columns are another element), and the Lemmings would simply walk on the top side of the image that resulted when composing all these elements (which could be placed freely). Slopes were tackled by looking left and right if the neighbouring pixel was > X pixels higher than the one the Lemming was standing on. If yes, the Lemming would turn around, if no it would walk on, following the slope. This way it was also relatively easy to make the environment destructable (by digging or exploding) and constructable (by building stairs).
The tools that were used to make Lemmings were shown in a 2019 GDQ talk, called Classic Game Postmortem: Lemmings [1]. The editor, and how fast the loop was between tweaking a level a little bit and being able to try it again, are pretty impressive. The talk also goes through the wonders of high quality low pixel animation.
While not a platformer, another example of a non-tiled, non-bitmap game is the Last Ninja series on the C64. At the start of each level you can see the graphics being built up by drawing/painting operations.
>that would have far exceeded the capacity of an Amiga disk
Simple bitmaps for game-mechanics/level design goes back to the 8-bit era.
Bitmaps with colour bytes that indicate level/enemy/goal hit points tend to compress rather well, since a lot of the bit-space is 00's, and .. anyway .. Amiga is legitimately viewed as one of the birthplaces of interesting compression for these kinds of reasons ..
