I see no indication as to why this was removed - from what I am collecting, it is still present in the native DLL and in the C-style header - the reference manual does not mark it as deprecated or similar.
V 1.8.7 seems to have some "special effects", such as stack imbalance (due to some inconsistency with calls) when called from managed code (for example, an open function) ... I have not tried but adding back H5F.flushin. The NET wrapper is pretty simple ... I would suggest doing it if you really need it ... IF this creates an even bigger problem than an unbalanced stack that can always be overcome by calling the "native" proxy DLL "with the correct conventions about calling and using this from the .NET shell.
EDIT - according to the comments:
Available .NET wrappers look rather weak, sometimes incomplete, etc. “Since the source is available, implementing a .NET wrapper seems like a doable task.”
As a starting point, see: