The AbuseFilterBypass extension was originally developed by and for Wikia; the default branch of Wikia's GitHub repo is not "master" but rather "dev".
Somehow this meant that when the history for the gerrit repo was imported from Wikia's GitHub repo in December 2019, the "dev" branch was set as the default instead of "master" for the GitHub mirror of the AbuseFilterBypass extension repository. This can be confusing for users using the GitHub mirror for whatever reason(s). The "dev" branch in reality should not exist at all and visiting the URL https://github.com/wikimedia/mediawiki-extensions-AbuseFilterBypass should show contents of the "master" branch instead.