This GitHub Action sets a pull request status to pending if the title includes "WIP".

An example workflow looks like this (switch to the <> Edit new file tab when creating a new workflow and paste the code below):

name: WIP
on:
  pull_request:
    types: [opened, synchronize, reopened, edited]

jobs:
  wip:
    runs-on: ubuntu-latest
    steps:
      - uses: wip/action@v1
        env:
          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}

Then to prevent PRs from being merged, enable the WIP (action) status check in your Settings > Branch > [Branch Name] > Protect matching branches > Require status checks to pass before merging

I don't plan to add more features to it. It's only 10 lines of code, a great reference action to build one that matches your needs :)

Apache 2.0