Bitcoin Core – Can I use P2PKH transactions via the script path in P2TR?

The ways in which UTXOs can be used are limited to what the output script allows, and the configuration available to other types of scripts is not transferable. So no, you cannot use P2PKH UTXO using P2TR rules.

If you hear about P2SH “wrapping” P2PKH, this may mean it uses P2PKH. script (OP_DUP OP_HASH160 <public_key_hash> OP_EQUALVERIFY OP_CHECKSIG) as a P2SH redemption script instead of a pure output script (example transaction). This is possible, but not useful because there is no reason not to hash the key and use it directly. You can do the same thing using the script path P2TR, but it’s also less useful.