After the Wellington upgrade, we disallow txs that don't have nVersion set to 1 or 2.
This is already a policy rule, therefore doesn't require changes by any wallet.
Forbidding the version field from having an arbitrary value (even if just for miners) allows us to use this field for a new transaction format, e.g. Mitra.
The Bitcoin Cash chain is doing the exact same upgrade, for more in-depth analysis (including which non-1-2 transaction versions have historically been mined already), you can [[ https://gitlab.com/bitcoin.cash/chips/-/blob/3b0e5d55e1e139046794e850287b7acb795f4e66/CHIP-2021-01-Restrict%20Transaction%20Versions.md | read their CHIP ]].
Inspired by [[ https://gitlab.com/bitcoin-cash-node/bitcoin-cash-node/-/merge_requests/1599 | bchn#1599 ]].