It appears the build-api effort at
https://github.com/cgwalters/build-api hasn't really caught on.
systemd appears one of the very few projects actually supporting it.
It does confuse certain tools though. E.g. debhelper by finding a
configure script wrongly assumes this is an autoconf project and thus
needs to be told explicitly that this is in fact a Meson project [1].
Given that Meson is an established build system by now, it appears ok to
drop this compat layer, which will never be fully complete anyway.