Remove SSZ unions

[tersec]

SSZ unions are not used and have never been used in any version of deployed consensus-layer Ethereum. Align spec with this.

[ralexstokes]

unions were used in very old versions of sharding specs, and its possible we could use them in future iterations of the spec

instead of deleting them, we could add a note that they are not part of a "core" or "minimum required functionality" SSZ implementation

[ralexstokes]

another data point:

Portal uses unions in their networking specs here: https://github.com/ethereum/portal-network-specs/blob/master/portal-wire-protocol.md

[leolara]

I am closing this issue because it seems stale. Please, do not hesitate to reopen it if this is a mistake

[tersec]

Yeah, I've been monitoring the relevant conditions for this.

The last real user was Portal, which is why I didn't take further action on it. But conditions around Portal changed.I

I'm not sure I can reopen, but it should be reopened.

[leolara]

Has this been discussed in an EIP? If we want to modify the SSZ standard, I guess the community should give feedback through the EIP process?

[michaelsproul]

We (LH) use union internally for data we store on disk. Having an SSZ type that corresponds to a tagged enum with arbitrary variants is very useful for us.

If deleted from the spec we will likely maintain our implementation as a non-standard extension.