How can I ensure that friends with iPhones send me SMS/text instead of iMessages? I have an android phone and normally use the Google Messages app for sending and receiving SMS.
Yesterday, I used the macOS Messages.app app for the first time to send a friend a link.  I guess they have an iPhone, because now when they text me it comes to Messages.app instead of to my phone.  I'm assuming this will be the same for all other iOS users.
How do I prevent all Messages.app and iOS Messages users from sending me NON-SMS messages?  Thinks I have tried:

Closing Messages.app.  I still get macOS notifications of messages.
In Messages &gt; Preferences… &gt; Accounts &gt; [my email address], untick &quot;Enable this account&quot;.  Then my friends messages disappear and I never see them at all, at least until I reenable in Messages.app.
Looking in System Preferences and on icloud.com and appleid.apple.com to see if there is some setting I can disable—but I've not found anything relevant.
Using Apple's Deregister iMessage tool to unregister my mobile number.  When I attempted this it gave me the message &quot;Your phone number is not registered with iMessage.&quot;

This question was closed as a duplicate of Removing phone number from iMessage, but as noted my phone number is not registered with iMessage, so that question is not relevant.
Am I doomed to never receive SMS from iPhone users ever again?
You may not have to do anything except deregister your cellular number from messages.

https://selfsolve.apple.com/deregister-imessage/

At that point, if people send you messages to your Apple ID they won’t deliver if you sign out of the app on any remaining Apple OS devices.