Same issue was faced so when searched in intervet got the solution saying

jobs:
  prepare-matrix:
    runs-on: ubuntu-latest
    outputs:
      platforms: ${{ steps.platform-matrix.outputs.matrix }}
      regions: ${{ steps.region-matrix.outputs.matrix }}
    steps:
      # Step 1: Retrieve and parse PLATFORMS
      - name: Platform Matrix
        id: platform-matrix
        env:
          PLATFORMS: ${{ vars.variablename }}
        run: |
          # Convert the repository variable to JSON array
          MATRIX_JSON=$(jq -nc --arg values "$PLATFORMS" '$values | split(",")')
          echo "::set-output name=matrix::$MATRIX_JSON"

Changed the set-output line with below line

echo "matrix=$MATRIX_JSON" >> $GITHUB_OUTPUT

The code ran perfectly . Same is also communicated here : https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/