Install Signal and tell your friends to do so as well unless they want to use SMS to contact you. If they refuse to use Signal in order to stay in touch, you should ask why. My guess is that those people are exactly the type you should have blocked years ago, so this is also a good opportunity to clean your address book and start from a clean slate.
Edit: That was badly worded, so here’s a clarification.
I’m not telling anyone to block anyone because of the app they use or don’t use. I’m talking about an underlying cause that affects many things such as the apps they use, flexibility and social interactions. Perhaps that person is actually a jerk, but you just haven’t realized yet. Here’s your opportunity to find out. Perhaps there’s a good reason for not installing a new app, but you just haven’t thought of it yet. That’s why you need to ask.
If someone doesn’t want to install a new communication app, that’s ok. Explain why WhatsApp sucks and how Meta is exploiting and abusing the users. Explain how Signal respects your privacy. If the answer is still no, then you have discovered something important. Maybe your friend is a bit more inflexible than you previously thought. Perhaps you value the friendship more than they do. Is it really worth it to be in contact with a person who doesn’t value you and the friendship that much? Maybe this is the type of person who you should have abandoned a long time ago. Not because of their app preferences, but because of their personality.
Of course there’s always a possibility that there is a valid reason and you’re not dealing with a jerk after all. If that’s the case, you can always use SMS and email to stay in touch.