Hi,

in recent LuaTeX versions \mathflattenmode has been marked as obsolete.
We would like to use this functionality in LuaLaTeX, so it would be great
if it could stick around. We only need it to disable flattening though,
so if you want to avoid keeping the whole thing we would also be happy
to just have a boolean "disable mth flattening" option equivalent to
\mathflattenmode=0.