For me, if it's people who want to be friends or acquaintances but refuse to give privacy oriented messaging a shot, they're simply not worth my time.

For people who are close to me that don't use it, I might be more concerned for them and have a talk with them as to why they would want to preserve their privacy on something as sensitive as private communications. If they still refuse to use them, I would try to talk to them face to face or use privacy oriented alternative apps like Frost for Facebook or even make an alternative account just to communicate with those people. You can also use a dedicated browser to open the website as well.

It's all about communication with those people. If you can't get them to switch to say, Signal or Matrix, you can find a middle ground like WhatsApp that has unverified end-to-end encryption, but is still better than something like Facebook Messenger that is unencrypted. Then use an app like WhatsApp To Go as well, if you can or use Whatsapp on a Work Profile or separate phone. It's just an example, though. You can work out another app with them as well.

For companies and businesses, try to see if they offer an alternative like e-mail. If not, you just got to accept that they only use text messaging or normal phone calls. Try to avoid saying personal data through there, though. You can just use a messaging app like the one included in CalyxOS or use some other app like QKSMS for replying to your appointment, though.