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
  https://support.apple.com/guide/iphone/block-filter-and-report-messages-iph203ab0be4/ios   
Preserving a Message
  If you want to save a message forever, make a screenshot (or
  PDF) of the message.
End-To-End Encryption (EE2E)
  https://en.wikipedia.org/wiki/End-to-end_encryption
  https://security.apple.com/blog/imessage-pq3/   
  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.
  
  Session - an end-to-end encrypted messenger that minimises 
  sensitive metadata    
    https://getsession.org 
    https://getsession.org/download
    http://arxiv.org/pdf/2002.04609
    https://apps.apple.com/us/app/session-private-messenger/id1470168868   
    
    Session is an end-to-end encrypted messenger that
    minimises sensitive metadata, designed and built for
    people who want absolute privacy and freedom from any form
    of surveillance.
     
  
  Signal - Secure Phone Calling & Text Messaging
    https://www.signal.org
    https://www.signal.org/download/macos/
    https://apps.apple.com/us/app/signal-private-messenger/id874139669
  
    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
    https://support.apple.com/guide/security/facetime-security-seca331c55cd/web
    https://support.apple.com/en-us/HT204380
  
    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    
    https://support.apple.com/guide/icloud/set-up-messages-mm0de0d4528d/icloud   
    https://support.apple.com/en-us/102651   
  
  
  As a point of comparison, neither Facebook Messenger nor
  Facebook WhatsApp are considered to be private or secure.   
     
     
     
 
    sam.wormley@icloud.com