The ExpandSubTypeCheckAtParseTime flag was introduced with JDK-8238691. It forces the expansion of sub type checks during parse time. This has an impact on some of the optimizations that only work with SubTypeCheckNodes. It requires some workarounds (most recent mainline example: JDK-8328702) and has a maintenance cost.

We've now found another issue in Valhalla (JDK-8331912) where flatness information cannot be propagated anymore when eagerly expanding sub type checks. There is no easy workaround.

Given all that and that the flag only found one real bug (AFAICT, JDK-8295414), we should remove it.