Managing Your Digital Stuff - Messages

Text messaging, like email, is an very useful communication
tool. As with email, messages can build up on our devices
without attention by the user. Apple has an option of
auto-deleting messages more than 30 days old. To the best of
my knowledge there is no way to sync deleted messages between
our devices.

iMessage Addressing:
  On Macs -- Messages > Settings > iMessage
  On iPhone/iPads -- Settings > Messages > Send & Receive
Identical settings on all you Apple devices is recommended.  

Blocking Unwanted Messages If the message is from a legitimate sender, such as a political party, dentist, or retailer, one can often reply with STOP and the sender will honor the request. Block, Filter, and Report Messages on iPhone Preserving a Message If you want to save a message forever, make a screenshot (or PDF) of the message. End-To-End Encryption (EE2E) End-to-end encryption (E2EE) is a system of communication where only the communicating users can read the messages. In principle, it prevents potential eavesdroppers - including telecom providers, Internet providers, and even the provider of the communication service - from being able to access the cryptographic keys needed to decrypt the conversation. Signal - Secure Phone Calling & Text Messaging Signal messages and calls are always end-to-end encrypted and painstakingly engineered to keep your communication safe. We can't read your messages or see your calls, and no one else can either. Apple iMessage And Facetime & Privacy We designed iMessage and FaceTime to use end-to-end encryption, so there's no way for Apple to decrypt the content of your conversations when they are in transit between devices. Attachments you send over iMessage (such as photos or videos) are encrypted so that no one but the sender and receiver(s) can access them. Apple iMessage Syncing in iCloud As a point of comparison, neither Facebook Messenger nor Facebook WhatsApp are considered to be private or secure.