This topic came up regularly during the KF6 port. IIRC the basic rule is that
things that can and should be co-installable should be versioned and things
that cannot or must not be co-installed are not. That however means this a not
always immediately obvious case-by-case decision.
Additionally
I've been scripting a task that uses various command line binaries
installed by either Frameworks, Plasma or Applications, and noticed that
there doesn't seem to be consistency in their major version suffix.
Some are installed only with a suffix "6", presumably to be able to be
coinstalled with the
