YAML template params documentation

[igordot]

I am trying to understand the _pkgdown.yml template field. Specifically, when is it necessary to use params?

In the docs, there are a few examples, such as:

template:
  params:
    google_site_verification: _nn6ile-a6x6lctOW

template:
  trailing_slash_redirect: true

The behavior also seems to change over time. With BS3, bootswatch had to go under params, but not with BS5. Is the proper usage with all the possible fields documented somewhere?