It isn't persistent. Signal always defaults to secure messaging, which caused me regular headaches when I used it. More than a few people I knew installed Signal, used it briefly (or not at all), and went back to sms. Signal would always default to sending securely, and I'd always send these people a message that they'd never see, because they were no longer using Signal.

After some time I'd realize they didn't get it and would have to resend as an insecure sms. Missed more than a few time-critical communications because of that.

I really, really wish they'd made it a per-contact default I could have set.