Sounds a bit invasive for back branches, but wondering if we couldn't just modify window_ntile_support() to reject any ntile args other than Consts. count(*), row_number(), rank(), dense_rank(), percent_rank() and percent_rank() all can't suffer from this issue as they don't have an argument. count(expr) would need to have something done to stop the same issue from occurring. Maybe int8inc_support() could just set req->monotonic = MONOTONICFUNC_NONE if the req->window_func has an arg, effectively disabling the optimisation for count(expr).
You're right that count(expr) also suffers from this issue.
select 1 from (select count(s1.x) over () as c from (select (select 1) as x) as s1) s where s.c = 1; ERROR: WindowFunc not found in subplan target lists
For back branches, the idea of modifying window_ntile_support() and int8inc_support() to reject any non-pseudoconstant args also seems reasonable to me. One thing I noticed is that sometimes it's not easy to tell whether the arg is pseudoconstant or not in the support functions, because a pseudoconstant is not necessarily being type of Const. For instance, count(1::text) is a CoerceViaIO, and ntile(1.0::int) is a FuncExpr. But these are very corner cases and I think we can just ignore them.
