Some repos does not support stacks. Which ones?

[josefs]

I wanted to try out stacked PRs so I created a stack for this repo: https://github.com/github/codeml-detector/
However, when I ran gh stack submit it reported that stacked PRs were not supported for the repo. Full output below
How can I know if a repo supports stacked PRs?

Pushing 2 branches to origin...
? Title for PR (branch josefs/xpiCollector): New collector for importing xpi data
✓ Created PR #1682 for josefs/xpiCollector
? Title for PR (branch josefs/xpiDataImport): Add benchmarks and triage data for the XPI dataset.
✓ Created PR #1683 for josefs/xpiDataImport
⚠ Stacked PRs are not enabled for this repository
✓ Pushed and synced 2 branches