user4273: I know that Bitcoin has many different transaction output types (P2SH, P2WPKH, P2WSH, P2TR, etc).
And I know that P2PKH is an old transaction output type that has existed since before Segwit and before Taproot.
Is it still possible to create a P2PKH output that works with Schnorr signatures?

user51948: No, Schnorr signatures are only used in taproot spends (v1 segwit with 32 byte witness program) as specified in BIP-342. This cannot be P2SH wrapped, and are encoded only with Bech32m.
Script path spending of Taproot modifies OP_CHECKSIG(VERIFY) to use Schnorr signatures instead of ECDSA. A new opcode is also made available for batch verification, OP_CHECKSIGADD. This means it is no longer possible to use ECDSA in taproot scripts.

user5406: No, it is not possible to use Schnorr signatures with old output types.
Current node software evaluating transactions expects an ECDSA signature for an input spending a P2PKH output. Putting a Schnorr signature there would be invalid per the accepted rules. Even if the rules were updated to allow using Schnorr signatures, old node software would always reject a transaction that uses the updated understanding, and this would therefore classify as a hardfork.

4273: <p>I know that Bitcoin has many different transaction output types (P2SH, P2WPKH, P2WSH, P2TR, etc).</p>
<p>And I know that P2PKH is an old transaction output type that has existed since before Segwit and before Taproot.</p>
<p>Is it still possible to create a P2PKH output that works with Schnorr signatures?</p>
 51948: <p>No, Schnorr signatures are only used in taproot spends (v1 segwit with 32 byte witness program) as specified in <a href="https://github.com/bitcoin/bips/blob/master/bip-0342.mediawiki#specification" rel="nofollow noreferrer">BIP-342</a>. This cannot be P2SH wrapped, and are encoded only with Bech32m.</p>
<p>Script path spending of Taproot modifies OP_CHECKSIG(VERIFY) to use Schnorr signatures instead of ECDSA. A new opcode is also made available for batch verification, OP_CHECKSIGADD. This means it is no longer possible to use ECDSA in taproot scripts.</p>
 5406: <p>No, it is not possible to use Schnorr signatures with old output types.</p>
<p>Current node software evaluating transactions expects an ECDSA signature for an input spending a P2PKH output. Putting a Schnorr signature there would be invalid per the accepted rules. Even if the rules were updated to allow using Schnorr signatures, old node software would always reject a transaction that uses the updated understanding, and this would therefore classify as a hardfork.</p>
