Docs at https://rainprotocol.github.io/rain.flow
V2 was deprecated to remove native flows entirely from the V3+ interface.
This is because flow implementations are likely to want to implement Multicall
like functionality, such as that provided by Open Zeppelin.
Multicall based on delegate call preserves the msg.sender which is great for
allowing EOA wallets like Metamask to flow under their own identity, but is a
critical security issue when reading msg.value (e.g. to allow native flows).
https://samczsun.com/two-rights-might-make-a-wrong/
V1 was deprecated because the SignedContext struct that it relies on was
deprecated upstream. SignedContext was replaced with SignedContextV1 which
was a minor change, reordering fields only for more efficient processing.