Older Galaxy smartwatches are getting One UI and a bunch of other new features plus optimizations. Rollout for each product varies by region, meaning not all users might get the new update at the same time.
Wearable Tech May 20, 2019
The Android 9 Pie update with One UI is now available to some Samsung Galaxy S9 and Galaxy S9 Plus devices in the United States. However, the rollout is limited.
Phones/Tablets/Mobile Tech January 29, 2019