RFC 117: Remove dated tags for epoch branches

rfcs/epochs_tags.md

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+# RFC 117: Remove dated tags for epoch branches
+
+## Summary
+
+Stop creating periodic tags for the `epochs/*` branches, such as
+`epochs/daily/2022-06-30_02H` created for `epochs/daily`. Also remove all such
+tags created so far.
+
+## Details
+
+[wpt#26122](https://github.com/web-platform-tests/wpt/pull/26122) introduced
+these tags in order to help identify which commit had triggered CI runs in the
+past. However, the `./wpt rev-list` command already supports listing those
+commits. This will be documented.
+
+## Risks
+
+The `./wpt rev-list` command depends on the `merge_pr_*` tags, but the CI job
+that creates them is not 100% reliable, see
+[wpt#23426](https://github.com/web-platform-tests/wpt/issues/23426). If the tags
+are missing but later backfilled, what `./wpt rev-list` lists can change, so
+that the commits listed aren't the ones that were pushed to `epochs/*` branches.
+In 2020 ~1-3% of tags were missing, and it continues to happen at least a few
+times a year. No mitigiation is suggested since backfilling tags is only done
+manually and rarely.
+
+Since these tags have been created since October 2020 there is a risk that
+someone has grown a dependency on them in their workflow. This RFC is mainly to
+make the change visible to uncover such uses.