I would suspect valve makers stopped making direct heated valves
because
the people who wanted to buy valves started buying - and preferred -
those
which were *not* direct heated.

I would suspect that the change was done entirely in terms of
convenience, rather than sound quality. The sound quality was no doubt
considered "close enough not to make a difference". And this is the
issue really - when even small changes in sound quality do make a
difference, then builders will use what they empirically find to be
better. No doubt nothing except a few volumes of statistics will have
any effect on your judgement, so it's pointless pursuing this,
particularly since you admit you know virtually nothing about DHTs.