it looks like d0794e5 lightly broke eip2718 abstraction by making type flags tx-associated instead of envelope-associated. We would now behave badly for networks where the type flag is different (e.g. TxEip1559 is flag 17, or where both 4 and 5 are TxEip7702

It also means that if we have a tx type with two different on different networks, they can't re-use the same codepaths

Need to move the flag encoding back into the envelope-level encode_2718 implementation

Originally posted by @prestwich in #1460 (comment)