In truth, the Galaxy Z Fold2 was never 'gone from Samsung's website', as some website's suggested. The Galaxy Z Fold2 remained live on Samsung's US website, just not its 'buy' page. Samsung has now corrected the error though, and is selling the Galaxy Z Fold2 again. However, the Galaxy Z Fold2 remains cheaper on Amazon (affiliate link).
No US carriers have Galaxy Z Fold2 stock in Mystic Black or Mystic Bronze, though. So, US carriers may have been running down their stock levels in anticipation of the Galaxy Z Fold 3. Conversely, Samsung still sells older flagships like the Galaxy Note 9, Galaxy S9 and the Galaxy S10e. Hence, Samsung has done nothing to suggest that it will stop selling the Galaxy Z Fold2 when it releases its successor.