Throughout 2015 and 2016 (per T95448, T107430 and related tasks) all first-party use of this hostname has been removed in favour of accessing resources through the canonical wiki domain names. At this point its traffic was also folded into the main "text" Varnish cluster. The "bits" Varnish cluster was removed. See Varnish#Cache Clusters.
bits.wikimedia.org used to be served by the Bits cluster (a dedicated pool of Apache servers). Since 2014 this separate cluster no longer exists and requests were now handled by the main app servers. This improved redundancy/spike proofing.
Entry points that used to exist on bits.wikimedia.org:
/:domain/load.php: Alias to the load.php endpoint of MediaWiki handled by ResourceLoader. Was originally put here for the benefits of a cookieless domain. Improved clients-side caching, HTTP/2 multiplexing, and added complexity of a separate domain were motivation to move it away from bits.
/beacon/*: For EventLogging and Statsv.
/geoiplookup: For Geolocation.
/static/*: Aliases to the /w/skins, /w/extensions/ and /w/resources/ directory of MediaWiki containing static assets. (T99096)