Google started sending out the update to Wear OS 5 for the original Pixel Watch and the Pixel Watch 2 back in September. However, shortly after the rollout started, a lot of people reported a very nasty issue with these older watches - following the update, the watch would be stuck on a blank screen. Google listened to all of the reports and halted the update's rollout on September 26, but it only confirmed this today in an announcement on its support pages for the Pixel Watch line. The company says it plans to resume the Wear OS 5 rollout for the Pixel Watch and Pixel Watch 2...
Source: https://www.gsmarena.com/google_confirms_it_halted_the_wear_os_5_update_for_the_pixel_watch_and_pixel_watch_2-news-64899.php