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.
#include <wallet/wallet.h>
#include <qt/receivecoinsdialog.h>
#include <qt/forms/ui_receivecoinsdialog.h>
#include <interfaces/node.h>
#include <qt/addresstablemodel.h>
#include <qt/optionsmodel.h>
#include <qt/platformstyle.h>
#include <qt/receiverequestdialog.h>
#include <qt/recentrequeststablemodel.h>
#include <qt/walletmodel.h>
#include <QAction>
#include <QCursor>
#include <QMessageBox>
#include <QScrollBar>
#include <QTextDocument>
ReceiveCoinsDialog::ReceiveCoinsDialog(const PlatformStyle *_platformStyle, QWidget *parent) :