The idea in the new manual will (I believe) be simply to tell people
to do slightly more primitive alignments, since I have found that
it's easier to write "#\hfil" than "\lft{#}". Furthermore, the
easier way doesn't seem harder to understand, and it gives TeX a
chance to do better error recovery in case of missing \cr. (Otherwise
the } after missing \cr is interpreted as the } that closes \lft, and
the error messages are less scrutable...)

Thus, I'm simply planning to leave \lft out entirely, and not to
provide a substitute.

\lft appeared in only one place of DVItype.web. That line should now be:

$$x=\left\{\vcenter{\halign{$#$,\hfil\qquad&if $#$\hfil\cr

Maybe I'll get a better idea when I actually write that chapter, but
for now all I am sure of is that \lft won't be used in the former way.