Last year, when Google first started rolling out its first-party RCS chat system, the company promised an easier way for folks to "deregister" their phone numbers in the event they switch platforms. Like iMessage, the system required a phone number-based registration, and that could mean missed messages if you forgot to disable the feature before changing to an unsupported device. Today that's no longer a problem: You can now deregister your phone number and disable chat with an easy form that's basically identical to Apple's system.
The change was first spotted by someone in the /r/universalprofile subreddit, though it doesn't seem to have been formally announced by Google. It's definitely new, but we don't know exactly when it rolled out, though we've been anticipating its release.
Pretty much exactly like Apple's iMessage deregistration.
The new system behaves exactly like Apple's. You enter the phone number you'd like deregistered from Google's Jibe/Chat/RCS messaging system, and that number is sent a verification code. (Note: that code gets sent whether or not you can even disable it for that phone number — just like Apple.) Once you enter your verification code, click/tap "verify," and either your number will be deregistered or you'll get an error explaining how/why it can't be. For example: If it wasn't registered in the first place, or if your number is associated with a carrier's RCS system instead.
The standard deregistration fallback without user intervention is around eight days, and while you can disable Google's chat much more quickly from directly inside the Messages app, that's not so easy to do if your phone is lost or stolen, or you simply forget to do it before you switch platforms.
Admittedly, I've run into way fewer issues with missed messages switching from Android and RCS to iOS and iMessage than vice-versa. I'm still missing communications being sent incorrectly to iMessage to this very day following our iPhone SE review, and Apple claims my number isn't registered — what can you do. But for folks making the switch away from Google's Messages app and chat, you can now disable it without needing a spare compatible Android device around and an inconvenient SIM swap.