There is (sometimes?) a way for Signal users to deregister from RCS which should presumably cause other devices to send messages via SMS instead of RCS.
People's Android devices can be opt in to receive messages via RCS based on phone number with either Google or their carrier. If the number is registered via Google, the number can be deregistered using a form. [1] For Verizon, it seems you can call Customer Support at 800-922-0204 to disable RCS. [2] Presumably other carriers have similar options.
Once RCS is disabled at Google/carrier level at the phone number level, other RCS compatible phones will fall back to SMS/MMS for delivering messages, which will cause the Signal app to be able to read messages via Android SMS APIs.
Disabling RCS, which apparently is encrypted with the Signal protocol for G messaging, so that people can continue to comfortably use unencrypted comms (sms) in Signal messenger itself would be a really wild course of action.
People's Android devices can be opt in to receive messages via RCS based on phone number with either Google or their carrier. If the number is registered via Google, the number can be deregistered using a form. [1] For Verizon, it seems you can call Customer Support at 800-922-0204 to disable RCS. [2] Presumably other carriers have similar options.
Once RCS is disabled at Google/carrier level at the phone number level, other RCS compatible phones will fall back to SMS/MMS for delivering messages, which will cause the Signal app to be able to read messages via Android SMS APIs.
Seems like quite the hassle to set up an SMS app.
[1]: https://9to5google.com/2020/06/19/google-messages-disable-rc...
[2]: "How do I turn off Advanced Messaging" https://www.verizon.com/support/advanced-messaging-faqs/