The definition of valid is different in different contexts. To old nodes taproot transactions are valid when they've been included in a block, but are invalid

The definition of valid is different in different contexts. To old nodes taproot transactions are valid when they've been included in a block, but are invalid when they are relayed on the network. This is accomplished by taking all the parts of the transaction that are reserved for future extensions and making their use invalid for the purpose of relay, mining selection, or wallet display in advance. So you can make a transaction that is invalid to new nodes, but valid-in-blocks to old but it'll still be invalid to them for other uses. It's sufficient that new stuff be considered valid in blocks by old nodes for the system to still come to consensus, since only the blocks are included in consensus. [Thanks for all the questions, BTW, it's been an interesting discussion.]