diff --git a/.github/workflows/scheduled.yml b/.github/workflows/scheduled.yml
index 96ef64005e5..e4afe6e993a 100644
--- a/.github/workflows/scheduled.yml
+++ b/.github/workflows/scheduled.yml
@@ -22,24 +22,68 @@ permissions:
   contents: read
 
 jobs:
+  # Figure out which branches to run CI on. This is the main branch, plus the
+  # three most recent release-X.Y branches (as per the support policy in
+  # RELEASES.md, we maintain latest, latest-1, and latest-2).
+  #
+  # Note this is based on branch existence rather than released versions, so
+  # from the moment a new release branch is created (at rc1, i.e. about two
+  # months before the .0 release) until that release is out, the oldest
+  # supported branch (latest-2) is not covered here. This is deliberate:
+  # testing the upcoming release is more valuable, and the branch in question
+  # is still tested on every PR and via workflow_dispatch.
+  prepare:
+    runs-on: ubuntu-latest
+    outputs:
+      branches: ${{ steps.branches.outputs.result }}
+    steps:
+      - name: Find branches to test
+        id: branches
+        uses: actions/github-script@3a2844b7e9c422d3c10d287c895573f7108da1b3 # v9.0.0
+        with:
+          result-encoding: string
+          script: |
+            const re = /^release-(\d+)\.(\d+)$/;
+            const all = await github.paginate(github.rest.repos.listBranches, {
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              per_page: 100,
+            });
+            const rel = all
+              .map((b) => b.name.match(re))
+              .filter((m) => m !== null)
+              .sort((a, b) => b[1] - a[1] || b[2] - a[2])
+              .slice(0, 3)
+              .map((m) => m[0]);
+            if (rel.length === 0) {
+              core.setFailed('No release-X.Y branches found.');
+            }
+            const branches = ['main', ...rel];
+            core.info(`Branches to test: ${branches.join(' ')}`);
+            return JSON.stringify(branches);
+
   trigger-workflow:
+    needs: prepare
     permissions:
       contents: read
       actions: write # Needed to trigger other workflows.
     strategy:
       matrix:
-        branch: ["main", "release-1.5", "release-1.4", "release-1.3"]
+        branch: ${{ fromJSON(needs.prepare.outputs.branches) }}
         wf_id: ["validate.yml", "test.yml"]
     runs-on: ubuntu-latest
     steps:
       - name: Trigger ${{ matrix.wf_id }} workflow on ${{ matrix.branch}} branch
         uses: actions/github-script@3a2844b7e9c422d3c10d287c895573f7108da1b3 # v9.0.0
+        env:
+          WF_ID: ${{ matrix.wf_id }}
+          BRANCH: ${{ matrix.branch }}
         with:
           github-token: ${{ secrets.GITHUB_TOKEN }}
           script: |
             await github.rest.actions.createWorkflowDispatch({
               owner: context.repo.owner,
               repo: context.repo.repo,
-              workflow_id: '${{ matrix.wf_id }}',
-              ref: '${{ matrix.branch }}'
+              workflow_id: process.env.WF_ID,
+              ref: process.env.BRANCH
             });