Sounds like a good time to remind people about Telegram having a similar function for quite some time now. And just yesterday they announced a feature to simplify logging into web sites using TG bots: https://telegram.org/blog/privacy-discussions-web-bots It might be a personal choice, but for stuff when privacy is really important I'd definitely pick Telegram over Apple, no matter how much the latter claims to keep…
Telegram is unencrypted by default. All standard messages are stored on the server. Telegrams secret chat mode (end-to-end encryption) uses home made cryptography, and has been panned by experts in past. All group chat is in the clear and stored on the server. This is not the case with imessage. Comparing Telegram to iMessage, telegram is not in the same league as Apple. I don't trust either from TLA's or well funded…
Chat archives are stored encrypted, not in plain text. Please cite your sources if you claim otherwise.