docs/data/reusables/actions/expression-syntax-if.md

When you use expressions in an if conditional, you may omit the {% raw %}${{ }}{% endraw %} expression syntax because {% data variables.product.prodname_actions %} automatically evaluates the if conditional as an expression. However, this rule does not apply everywhere.

You must use the {% raw %}${{ }}{% endraw %} expression syntax or escape with '', "", or () when the expression starts with !, since ! is reserved notation in YAML format.

Using the {% raw %}${{ }}{% endraw %} expression syntax turns the contents into a string, and strings are truthy. For example, if: true && {% raw %}${{ false }}{% endraw %} will evaluate to true.