How do I stop iMessage forwarding on my iPad after switching to Android?

You may need to deregister iMessage if you are using the same phone number on your new phone.

Deregister and Turn Off iMessage - Apple Support

It may take a couple of hours according to the information provided at the bottom of the page you used to deregister iMessage.

If the person sending you the message is not using your phone number and instead sending to your Apple Account using iMessage, then that Message will only appear on your iPad. On the iPhone you can send a message to either the Apple Account or phone number, so you will want to make sure they are using your phone number.

Deregister iMessage on your iPhone or online - Apple Support

Deregister and Turn Off iMessage - Apple Support

khayyam80 wrote:

Do you have any recommendation for where I can learn more about this. Or as to why apple decided to create such a system?

• Texts using iMessage can be sent over Wifi, so you would still get them if no cellular connection.
• No Messaging charges from your cell provider. Not as important now with unlimited texting, but there was a time you had to pay for each one.
• End to End encrypted. An SMS message is not end to end encrypted and can be intercepted.
• Support for Read Receipts and recipient typing indicators.
• Allows iPad to use iMessage, since no cellular connection is needed and the phone number linked to the Apple Account has been registered.

By registering your phone number with iMessage, you get these benefits automatically without the sender of the message specifying your Apple Account and can simply send them to your phone number as usual. If you still have the device, you just need to turn off iMessage in Settings, or Factory Reset the device. You had to deregister because you had another device that was still using iMessage that was linked to your Apple Account.

See About iMessage on iPhone - Apple Support