Pre-taproot nodes (and miners) will not relay or mine taproot spends.
I forget the detail of backward compatibility, so i'll just ask it. Why pre-taproot won't relay taproot-transaction? Is it because the transaction missing witness and the (empty ?) script automatically treated as valid script?
There's nothing missing in the transaction, thanks to segwit. Versions 1-16 segwit are non-standard for older clients, hence not relayed. Version 1 segwit would become standard for newer clients (0.21.1 and up) after block 709632.