Way back in 2015 Netflix announced it would team up with Amazon, Google, Intel and other companies to develop a royalty-free codecs that worked well on modern devices. Two years ago they revealed the fruits of their labor as the AV1 codec, that promised not only freedom from licensing payments they wanted to avoid, but also a saving of up to 30 percent on the amount of data used to stream 4K HDR video. Now, Netflix has started using AV1 on a "select" set of titles, and only for users on Android who've enabled the "Save Data" feature.
One reason for limiting the use of AV1 for playback is likely a lack of hardware support, which could mean people will use more battery life to watch the same videos. You can already enable AV1 instead of VP9 on YouTube videos, under the settings for Playback and Performance, but it warns users that using it for anything above 480p will require a powerful computer.
While Netflix says it still plans to roll out AV1 across all platforms eventually, starting with data savings on mobile follows its 2016 switch to VP9, and the shot-based encoding technique that improved things so much it re-encoded its entire library in 2018. According to Netflix, AV1 shows "20% improved compression efficiency over our VP9 encodes." There's no word on which shows you should watch to find the new codec in use, but one way or another it's a huge leap beyond the days of VC1, Silverlight and 720 x 480 res videos that maxed out at 3,400 kbps.