So for WEBP we just added support for it to be treated like an image and do no transformation on it, because it’s widely supported nowadays. Meanwhile for HEIF we added an unconditional transformation to JPEG, since it isn’t supported in any browser whatsoever.
There is no precedent, in Discourse, of keeping the image in multiple formats and putting all in a picture srcset thing. While it’s doable, it’s also quite a bit of work.