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" )
...
2. address_type    (string, optional, default=set by -addresstype) The address type to use. Options are "legacy", "p2sh-segwit", and "bech32".
...
```

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.
QDialog(parent),
    ui(new Ui::ReceiveCoinsDialog),
    columnResizingFixer(nullptr),
    model(nullptr),
    platformStyle(_platformStyle)
{
    ui->setupUi(this);
    if (!_platformStyle->getImagesOnButtons()) {
        ui->clearButton->setIcon(QIcon());
        ui->receiveButton->setIcon(QIcon());
        ui->showRequestButton->setIcon(QIcon());
        ui->removeRequestButton->setIcon(QIcon());
    } else {
        ui->clearButton->setIcon(_platformStyle->SingleColorIcon(":/icons/remove"));
        ui->receiveButton->setIcon(_platformStyle->SingleColorIcon(":/icons/receiving_addresses"));
        ui->showRequestButton->setIcon(_platformStyle->SingleColorIcon(":/icons/edit"));
        ui->removeRequestButton->setIcon(_platformStyle->SingleColorIcon(":/icons/remove"));
    }
    
    QAction *copyURIAction = new QAction(tr("Copy URI"), this);