It is only available for enterprise (which is unreasonably expensive: 21usd vs 4usd per month per seat). We asked our devops to develop something similar using our existing ci infra, took them a few days.
I looked into that. Basically a GitHub action that goes over all of the prs that marked as "automerge" and updates them to master.
It is not a perfect solution, but it works perfectly well for our monorepo.