I wish they would use ANYTHING besides SMS.
Signal, whatsapp (not great but still better than being on my phone bill), snapchat is too much stupidity and spam.
I had one that used Telegram - it was nice for a change. It's not always private, but as long as it's not easy to get (like from a PI or police), I don't really care.
One provider had her Signal listed, but she stopped using it and never responded. Have to go back to SMS with her.
I use a (or more) google voice number. It's easy, doesn't show on phone bill, says it deletes stuff forever, but it's google, so you never know. I delete just in case phone falls into wrong hands. If they get a court order, I probably have bigger things to worry about.
Google Voice is regular SMS so I don't have to explain to someone that may or may not be technical, what to use and how to install / use something different.
The one big issue that sucks is that providers probably keep all their emails and sms on their phone. If the phone is looked at by someone or LE, if you're in their contacts, it's not going to end well.