They already did - WebP added a lossless mode and VP8 was updated to VP9.

Though the same may happen to JPEG; it always had 10-bit and 12-bit modes but most decoders don't support them. (Not sure if they can decode it as 8-bit or not.)

Are those breaking changes? I'd assume that a decoder would continue to work, just would not support the lossless mode/ VP9.

It will work on old files (since they've already been encoded) but not on new files with the same file extension, and not on, say, YouTube.

Interesting, thanks.