A backstop push is one that runs all tests and tasks to ensure there are no regressions on said push. They are used to e.g, determine whether the tree state is good enough to merge to central, or release Firefox.

It would be helpful if treeherder could display some kind of visual indicator to denote backstop pushes (e.g maybe a badge similar to the Push Health one).

Backstop pushes can be determined by reading the parameters.yml artifact from the decision task of a push. Alternatively they can be found in the taskcluster index, e.g:
https://firefox-ci-tc.services.mozilla.com/tasks/index/gecko.v2.autoland.latest.taskgraph/backstop

(currently there's only an index for latest, but we could add one for revision too if it helped matters)

Solving this would make the lives of sheriffs a bit easier, and would allow us to make backstops a bit less predictable (i.e, not necessarily run on pushid % 20).

This would not be too hard. Here is where we fetch the hg push information we need. We could get this extra info at this time:

https://github.com/mozilla/treeherder/blob/1250108abea7311f7161169b0f8dedb43f664556/treeherder/etl/push_loader.py#L278

Persisting it would likely require a DB migration, which is do-able. The Push table isn't as hard since it's smaller than Jobs. So the migration would take a reasonable amount of time, I suspect.