It is not taboo, but there isn't a lot of interest in the issue, and I
don't have time to do the digging you require, and I am not sure that
there was even a final writeup. Most of the people who were interested
in the past are inactive at this point.
This was a question that largely was satisfied over 25 years ago. We
satisfied ourselves to a level that met our needs at the time, and no
one has come up with counterexamples. The few problem areas, if they
can be called such, had to do with possible extensions to the language,
none of which are even being used to my knowledge.
I do recall that at one time we attempted to write a software exhaustive
test something like what was suggested by Mr Errington, but we decided
that the number of possible cases was far too large and hence
non-computable, given the PC hardware then available, and went to a more
logico-analytical approach. But any notes that I might have kept are
rather deeply buried in my archived files.