I go back to 1999.
I still have my first activity from 2004 recorded with a Forerunner 201. Then, the site was called MotionBased. All 4,504 activities are still there.
I still got my imported and phone tracked stuff from 2014 in the list.
If you've been syncing your activities all along, you shouldn't be missing any data. If you are, there is a possibility that you've logged into Garmin (Garmin Express, Garmin Connect Mobile, or Garmin Connect Web) using different credentials at some point - Data is Missing from My Garmin Connect Account
All my data go back to May 2017, from then on I imported Fitbit data into a Garmin account without having a Garmin watch.
In October 2019, I started using my Garmin fenix 6S Pro.
Remember that some features/data only became available later on.
Are you shure you never changed accounts like Seilogramp suggested.?