The question here is difficult. My initial thinking was that the rename was unnecessary. Eventually, I added the Provides as a compromise.
The issue is that there are third-party scanner driver .deb packages that won't be quickly re-compiled against the new sane-backends. I do see that there are symbols dropped in the new version, but it's difficult for me to tell whether those symbols are used by third-party drivers. Some libraries have way too many public symbols and many of the symbols aren't considered part of the API so dropping or changing them isn't a serious problem. libsane1.symbols.amd64 is over 7300 lines long so I guess this could be the case here. A typical way we can verify whether the dropped symbols are worth worrying about is to use https://codesearch.debian.net/ but that doesn't help here since most of this library's users are outside Debian. One more point: Ubuntu 18.04 LTS shipped with the bumped soname and the Provides. That means that any driver packages built against Ubuntu 18.04 will depend on libsane1 instead of the old libsane name. In that case, maybe the current package with the bumped soname and Provides is acceptable for Buster too. Thanks, Jeremy Bicha
