What about instead investing in XCVB?

As much as I would like to have something simplify my life, it is not my choice to use one system or another. I was suggesting something that might help rationalize the current library situation.

Such enforcement will necessarily introduce backward incompatibility and pain,
which I think goes contrary to the goals of ASDF.

I am not talking about something that HAS to be enforced, but that it can be optional. It is by no means a good coding practice to put things in the *.asd file that do not belong in it. Promoting this message from the ASDF development forum is not wrong by itself and need not cause any pain at all -- a warning message somewhere in the build, explaining the situation of the libraries in the system does not cause such a panic or disrupture, does it?

I think it should be spoken clearly here about what is expected also from the ASDF maintainers. Either you envision it as a hack until something better comes along, or you consider the possibility of gradually evolving towards something better.

As to systems that currently use weird ASDF extensions,
you could either make XCVB's ASDF converter better,
or just convert these systems by hand.