Question : was there some fundamental reason not to support union types?

[balayanv]

In most projects one often needs a wrapper type that can carry one message from set of predefined types.
In all codecs where this is not supported people end up doing this in code (writing an int, and mapping that int to a type etc...)

Was there some fundamental reason for SBE not to have this? maybe its some sort of anti pattern...

I'd imagine that things like event logs in event sourced systems are streams of wrapper types that can carry many different types of messages rather than one type with bunch of optional fields...