I can’t find it right now, but the Debian docs explicitly indicate that you shouldn’t do that because the internal format is not stable. It has changed significantly in the past, and the only “supported” way to do it is with the official Debian tools
The .deb format doesn't change significantly at all, the only changes in recent years have been changes to default compression choices (old ones are retained forever though) and some additional fields in the control file.
The only significant future change I can think of is the mtree stuff, which would add an additional metadata file to .deb files.
There are several warts in the .deb file format that the dpkg folks aren't fixing because of compatibility concerns.
