Apple Watch 4 can not delete call history

You are correct. The calls must automatically delete as new calls are added (just like text messages0. To prove this to yourself tap on "Phone" on your watch...then tap on Recents....scroll down to the bottom. On my Watch the last call s listed go back to March 2019.

View and delete the call history on your iPhone

1. Open the Phone app, then tap Recents to view a list of all recent calls.
2. Tap Edit.
3. Delete your entire call history, or a specific call. To clear all history, tap Clear, then tap Clear All Recents. Or, to clear a single call, tap Delete , then tap Delete.