Merging, as the very creation of this feature branch resulted in the intended workflow of this PR.

Does the exit 1 make the workflow register as failed and send everyone emails?

Does the exit 1 make the workflow register as failed...

Yes, I was testing this way of filtering branch creation events, using the needs: github_properties in the subsequent job of this workflow. I've since removed this and just put it in as a conditional.

... and send everyone emails?

Only if you have notifications configured to do so.

See also my reply to your earlier comment. This conditional is only required because I'm triggering the workflow with:

on:
  push:
    branches:
      - master
      - maintenance/*
  create:
    tags:
      - 'v[0-9]+.[0-9]+.[0-9]+*'

I do this because the attributes for tag creation events are more explicit than push events. See 'Aside' in #8.

The triggering context of GitHub Actions is rather surprising/unexpected, e.g. the above triggers off branch creation events as well. The need for the conditional (now preceding the main job and not resulting in a failure) could be avoided altogether by only triggering off push events, but this requires further assumptions about branch names and reference text.