Another addition in GitLab 13.7 are the rules:variables. This allows some logic in setting vars:

job:
  variables:
    DEPLOY_VARIABLE: "default-deploy"
  rules:
    - if: $CI_COMMIT_REF_NAME == $CI_DEFAULT_BRANCH
      variables:                              # Override DEPLOY_VARIABLE defined
        DEPLOY_VARIABLE: "deploy-production"  # at the job level.
    - if: $CI_COMMIT_REF_NAME =~ /feature/
      variables:
        IS_A_FEATURE: "true"                  # Define a new variable.
  script:
    - echo "Run script with $DEPLOY_VARIABLE as an argument"
    - echo "Run another script if $IS_A_FEATURE exists"

Another addition in GitLab 13.7 are the rules:variables. This allows some logic in setting vars:

job:
  variables:
    DEPLOY_VARIABLE: "default-deploy"
  rules:
    - if: $CI_COMMIT_REF_NAME == $CI_DEFAULT_BRANCH
      variables:                              # Override DEPLOY_VARIABLE defined
        DEPLOY_VARIABLE: "deploy-production"  # at the job level.
    - if: $CI_COMMIT_REF_NAME =~ /feature/
      variables:
        IS_A_FEATURE: "true"                  # Define a new variable.
  script:
    - echo "Run script with $DEPLOY_VARIABLE as an argument"
    - echo "Run another script if $IS_A_FEATURE exists"

It can also be done on workflow level for the whole pipeline:

workflow:
  rules:
    - if: $CI_COMMIT_REF_NAME == $CI_DEFAULT_BRANCH
      variables:
        DEPLOY_VARIABLE: "deploy-production"
    - if: $CI_COMMIT_REF_NAME =~ /feature/
      variables:
        IS_A_FEATURE: "true"

Then consider though that workflow rules affect the whole pipeline. You'll need to cover all your use cases.