Considering how Taproot and other things are part of the Bitcoin protocol, any fork that doesn't have it is not exactly Bitcoin anymore.
But the fork could be based on older Bitcoin Core version, which doesn't contain Taproot support.
--snip--
Since you've written that no one can change your mind:
Use Bitcoin Core v0.20.2 which doesn't include TapRoot implementations (including test networks),
but its last patch was 3 years ago so use it at your own risk.
That client wont relay transactions that has TapRoot inputs but will still accept the blockchain like a normal Bitcoin node would.
And its GUI/RPC supports PSBT, but quite outdated.
It's also worth to mention since that version, there are few updates to PSBT feature[2-3].
[1]
https://github.com/bitcoinj/bitcoinj[2]
https://bitcoincore.org/en/releases/0.21.0/[3]
https://bitcoincore.org/en/releases/25.0/