With an 18-pin DIP, the first thing that came to mind was PIC1654, but the pinout doesn't match.
1 batt- 2 LED 3 switch 4 switch 5 switch 6 switch 7 diode to batt- (reset?) 8 batt+ 9 LED 10 cap to batt-, resistor to batt+ (oscillator?) 11 LED 12 piezo 13 piezo 14 switch 15 switch 16 switch 17 switch 18 LED
Comparing two 4-bit inputs and lighting LEDs doesn't require a lot of smarts, and playing simple tunes wouldn't take much more.
Later Simon games used a custom chip, but they sold tons of those over many years. It was likely cheaper to use an MCU for a lower-volume game. Ideal Maniac used a PIC.
There might be more markings on the bottom of the chip.
I'd be happy to decap and photograph it if you want, but it will destroy it.