I see that legacy means P2PKH in both command line options and RPC help info:
-addresstype
What type of addresses to use ("legacy", "p2sh-segwit", or "bech32", default: "p2sh-segwit")
getnewaddress ( "label" "address_type" )
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2. address_type (string, optional, default=set by -addresstype) The address type to use. Options are "legacy", "p2sh-segwit", and "bech32".
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The useLegacyAddress checkbox describes itself as “Generate legacy address” - however, it actually makes the wallet GUI generate “p2sh-segwit” address for the user if it's checked.