Sorry, but it's not clear to me: is the only change for inputenc to
complain in the case of xetex and luatex? I hope so. If that's the
case, then it's all relatively minor; if you're changing behavior with
pdftex/etex, that's another matter entirely.
At any rate, as a matter of principle, MHO: I agree with Heiko that at
this late date, it would be much better to make the new checks be
warnings rather than errors. Otherwise documents which produced output
before will no longer do so. This seems to go against the fundamental
mandate of "be backward compatible" for latex2e.
I'm not sure if this is happening, but if it is: I feel even more
strongly about not "invalidating" any new characters under any
circumstances whatsoever. That is also just breaking users for no good
reason, as far as I can see.
Finally: we are only a few weeks from putting this out into the world.
It seems to me that anything beyond a warning if xetex/luatex are being
used is too invasive a change to be contemplating at this point. Most
users who will be affected by the new code will not see it until much
later. If you must legislate inputenc user behavior (you can tell by my
phrasing that I am not in favor of the idea, not at all), I think it
would be much better to do it after the release, when the only users who
will be affected are those who are asking for the bleeding edge, and
when you can quickly make a follow-up release to fix those users, as
When Frank told me about plans to make a new latex2e release for TL14,
this is not the kind of thing I was imagining would be coming up.
Sorry for the rather vehement message, but ... I'm scared.