Which is why dumb phones and feature phones aren't common anymore, and the people choosing them are specifically choosing it to avoid being available via WhatsApp/Signal/Slack/Discord/Teams/whatever else.

My FIL for example has a clamshell feature phone, because he doesn't want to be reached except by phone or SMS. He doesn't want to read email or get messages on his phone, he wants to restrict that to when he's in front of his computer.

So yes, you would not be able to use messaging clients on a dumb phone, that's the idea behind their use today.