Naven
Answers
What is the easiest messaging app for an older relative?
One with nothing to install and no account to create in an app store. A link that opens in the browser they already have removes the two steps that most often defeat the attempt.
The failure is rarely the messaging itself. It is the app store password nobody remembers, the storage that is full, and the account creation form on a small screen.
A web-based tool skips all three: you send a link, they tap it, and they are in the conversation. Signing in is an email address and a code.
The second thing that helps is a large, quiet interface without a feed competing for attention.