https://www.cnbc.com/2018/06/06/mueller-team-zeroes-in-on-encrypted-apps-as-witness-turn-in-phones.html
I tried to research whether or not texts using apps like Dust, that delete your messages after they are read (or within 24 hours), can be recovered. An article from 2015 says yes; nearly all messages were recovered by a forensics specialist. This news debunked the information present on their website.
Whether or not things have changed since then, I couldnt find a more recent article. You'd have to assume that its likely or they wouldnt be asking for their phones.