Great to see you still here, David!
I might be missing something here because I highly doubt any user has a record of their texting older than their phone. Even purpose-built texting services like WhatsApp or Messenger have trouble storing and fetching truly old messages (I have a 2-year hole in my Messenger conversation with a friend), let alone native texting apps without a centralized message storage server.
Good point. The spreadsheet template I uploaded to Google Sheets is based on my Open Document .ods expense tracking file, which has been functioning offline for over 2 decades, initially via OpenOffice, now through LibreOffice. It can be downloaded from Google Sheets back as an .ods file, so I added a note about that to my post.