Aren’t there unofficial extensions to mp3 for gappless playback?

Yes and no.

IIRC an MP3 track is divided in fixed-length frames and unless the actual audio matches perfectly with the end of a frame, it’s not possible and that’s why cross-fading plugins for audio players were invented. The padding data is there either way but can be documented in the metadata section of a file.

Last I checked (and that was years ago, so I may be wrong) this approach was never perfect and prone to breaking. It’s an inherent flaw with the format where some form of workaround exists.

That said, for most use cases this is irrelevant.