How can I delete pinned timers on my Apple Watch?

Sorry, but the default watchOS timers can not be removed.

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copied from: Set timers on Apple Watch - Apple Support

If you create your own (custom) timers and pin them on your Timers screen, the default timers will move downwards, out of sight.

However, if you use the Feedback link, you can tell Apple directly about your wishes for future watchOS updates.

This is the only way to reach them, as Apple does not follow theses user to user communities.

Feedback - Apple Watch - Apple

We are all fellow users here and have no insight about Apple's decisions on how much or when users can change the default watchOS settings or pre-installed apps.

I pinned my custom timers to a widget in the Smart Stack, so I see and select only my most needed ones.

You're welcome 🙂

The more users tell Apple about their wishes for changing this behaviour, the more likely those changes will be.

Have a look at this part in the Apple Watch User guide:

See timely widgets in the Smart Stack on Apple Watch - Apple Support