How would a new format be backwards-compatible? At least JPEG-XL can losslessly compress standard jpg for a bit of space savings, and servers can choose to deliver the decompressed jpg to clients that don't support JPEG-XL.

Also from Wikipedia:

Computationally efficient encoding and decoding without requiring specialized hardware: JPEG XL is about as fast to encode and decode as old JPEG using libjpeg-turbo

Being a JPEG superset, JXL provides efficient lossless recompression options for images in the traditional/legacy JPEG format that can represent JPEG data in a more space-efficient way (~20% size reduction due to the better entropy coder) and can easily be reversed, e.g. on the fly. Wrapped inside a JPEG XL file/stream, it can be combined with additional elements, e.g. an alpha channel.

It requires neither of those upgrades though? Unless you're still using Windows XP I guess for some reason. It's just an update to the image decoder