What happens if there is a problem with a template? You will have to disable the experiment to be able to use that template as these changes affect all templates, even if they were created in the past. And we can't break dogfood.

An alternative is to make Parameters opt-in per user.

Reminder to remove escape hatch after this feature is bullet-proof.
Question: is this escape hatch just useful for Coder or also our users? Our users as well.

Kirby: the escape hatch makes most sense to opt-out in the form itself and not in user settings and is also simpler to implement, e.g. a "Return to Old Experience" and store this preference in local storage.
Old page should have 'try new experience' button so you can opt back into parameters feature after you've opted out.

Is this a setting per template or is it site-wide? Complexity is trivial.
Decision: on a template basis per each user. They can revert per broken template which helps us understand which templates are broken.