Bug 1423215 made tasks upload both the old and new style runnable jobs file, then bug 1494750 added Treeherder support for the new style file. Once enough time has passed (eg so that the majority of try pushes have a base that includes the changes in bug 1423215), support for the old style file can be removed from Treeherder. The old file handling parts that can be cleaned up are here: https://github.com/mozilla/treeherder/blob/e69b3f01804de91028cbe7b4e263f4977ff45699/ui/models/runnableJob.js#L16-L25 https://github.com/mozilla/treeherder/blob/e69b3f01804de91028cbe7b4e263f4977ff45699/treeherder/etl/runnable_jobs.py#L37-L47 https://github.com/mozilla/treeherder/blob/e69b3f01804de91028cbe7b4e263f4977ff45699/treeherder/etl/runnable_jobs.py#L67-L79 In addition: * fetch_json()'s support for `force_gzip_decompression` can be removed (https://github.com/mozilla/treeherder/blob/e69b3f01804de91028cbe7b4e263f4977ff45699/treeherder/etl/common.py#L35-L43) * the runnable jobs API can be removed: - https://github.com/mozilla/treeherder/blob/e69b3f01804de91028cbe7b4e263f4977ff45699/treeherder/webapp/api/runnable_jobs.py - https://github.com/mozilla/treeherder/blob/e69b3f01804de91028cbe7b4e263f4977ff45699/treeherder/webapp/api/urls.py#L31-L35 * Several parts of runnable_jobs.py and SETA can be simplified, since they will only ever be fetching the most recent task ID

Stats for the fallback use can be seen here: https://insights.newrelic.com/accounts/677903/explorer/events?eventType=runnable_jobs_fallback&duration=604800000