@Mer__edith
There are obvious countermeasures both to "client side scanning" and to backdoored apps.
If client-side scanning get pushed to phones in general, temporarily you could use an older unlocked phone and block all updates by removing the updater under root or ADB.
When that dies, buy an unlocked phone supporting Lineage OS (successor to CyanogenMod) or Graphene.
Neither will ever support client-side scanning.
If you cannot obtain unlocked phones with unlockable bootloaders, start with a cheap "unlocked" phone not provided by any carrier. Use ADB to remove all known spyware, and use something like Tracker Control to deny all untrusted or unused apps plus the core operating system access to the Internet. That will keep the client side scanners from ever phoning home even if they are buried where you cannot find them.
Had Apple proceeded with their failed client side scanning plan, removing iPhotos and disabling cloud backup would have been enough to defeat it. Photos shared over Signal would have been completely invisible to Apple's phone nannies had that gone through.
BTW, Signal has already made it plain they will not comply with backdoors or client side scanning proposed by several countries, vowing to remove all "business presence" in any such nations instead. Signal will also refuse to block connections from foreign countries banning the service. Thus governments like Russia that ban signal find they have to attempt to block it themselves, with mixed results. Kicking it out of the Google Play store does little.
If your country finds a way to block Signal, connect to it over Tor. If they find a reliable way to block that too, it gets hard to conceal WHO you talk to, but you can still totally conceal WHAT you are saying.
This would be done the old SHAC way: local encryption and decryption using GPG, legally or otherwise. GPG is a program not a service, so no server is required anywhere. It could be passed around on flash drives if ISP's somehow find a way to block distribution of illegal software. Type your message on a text editor, cut and paste it into GPG, encrypt, then cut and paste the cyphertext into email or even SMS. Reverse to decrypt an incoming message.
Obviously in this scenario you must defy any key disclosure orders (as SHAC did in the UK) or be considered a snitch.
To defeat this tactic, carriers would have to be required by law to block any data they cannot read, and nobody is yet proposing that. Most of them (maybe all?) cannot even read https content and thus can only log your IP address, phone, and plain SMS histories. Even Facebook would have to be banned-and due to encrypted logins and payment processing online shopping and banking would also have to be banned.
These can be combined if governments try app bans AND client-side scanning: GPG on a phone with client-side scanning forcibly removed, and any comms protocol that works on a doctored phone.
So long as your text editor cannot be scanned, your screen cannot be scanned or keylogged, and your GPG setup cannot be scanned, you could conceal all the content of your comms though not the recipient with nothing more than plain SMS.
In short, the people the government REALLY wants to spy on will be the least affected by backdoored apps and client side scanning, low level dissidents, opposition party members, and legal journalists may be the most affected by it.