It's straight-forward enough to do in back-end code, to just reject a query if parameters are missing, but I don't think there's a way to define a schema that then gets used to auto-generate the documentation and validate the requests. If the request isn't validated, then the back-end never sees it.

For something like https://freamon.github.io/piefed-api/#/Misc/get_api_alpha_search, the docs show that 'q' and 'type_' are required, and everything else is optional. The schema definition looks like:

/api/alpha/search:
    get:
      parameters:
        - in: query
          name: q
          schema:
            type: string
          required: true
        - in: query
          name: type_
          schema:
            type: string
            enum:
              - Communities
              - Posts
              - Users
              - Url
          required: true
        - in: query
          name: limit
          schema:
            type: integer
          required: false

required is a simple boolean for each individual field - you can say every field is required, or no fields are required, but I haven't come across a way to say that at least one field is required.