I don't think you can have it both ways.

If currently, you can have extensions *without* having
EXTEND=T in the initial header, then by the "once FITS,
always FITS" rule, EXTEND=T has no meaning. To me, it
originally had value because I didn't have to figure out
where the next header was (I just returned an error if
that was attempted without EXTEND=T), but if you've allowed
other uses, you have to live with it and just create more
headache for the software developers.

Deprecate it. I agree with Arnold and others.
Arne