It's currently possible to reset customisations to a template whilst editing it via the dropdown in the top bar:

We may remove this dropdown in the Appearance Editor (#36207), but it should however still be possible to reset template customisation in this view, so we should explore ways we can make that possible.

One idea would be to add an actions dropdown in the Template tab of the inspector:

Additionally, it may be worthwhile making this action available on the list views at Appearance > Templates and Appearance > Template parts.