Some serializers (specifically MessagePack-CSharp) can't read from a Span, but they have no problem reading from Memory. This is already possible using this code:

Memory<byte> myData = reader.RawData.AsMemory(reader.Position, reader.AvailableBytes);
// "reader" is a NetDataReader

Therefore, adding in GetRemainingBytesMemory() is unnecessary, but it would be a nice addition just for completeness. I guess it would be added somewhere around here: