Matthias, to tell the truth, I was never sure about the soname.
I read this:

   https://www.debian.org/doc/debian-policy/ch-sharedlibs.html


"""
The SONAME and binary package name need not, and indeed normally should not, change if new interfaces are added but none are removed or changed, since this will not break binaries linked against the old shared library. Correct versioning of dependencies on the newer shared library by binaries that use the new interfaces is handled via the symbols or shlibs system (see Dependencies between the library and other packages).
"""

I took the interface to mean ABI, which did not change.  Also this:


"""Every time the shared library ABI changes in a way that may break binaries linked against older versions of the shared library, the SONAME of the library and the corresponding name for the binary package containing the runtime shared library should change. Normally, this means the SONAME should change any time an interface is removed from the shared library or the signature of an interface (the number of parameters or the types of parameters that it takes, for example) is changed. This practice is vital to allowing clean upgrades from older versions of the package and clean transitions between the old ABI and new ABI without having to upgrade every affected package simultaneously.
"""


So I left the soname at 2.