(IGNORE EVERYTHING BELLOW THIS AS IT’S THE ORIGINAL DEVELOPMENT WORK)
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Everything from this point down is out of date as of: 3-30-2020
This was my initial “netflix on the raspberry pi 4 development” blog post. Leaving it on here due to the comments, initial work, info for those interested, but I highly recommend using the easy method above (linked).
Last libwidevine extract: 3-29-2020 – v.4.10.1610.6 of libwidevine – EVERYTHING CONFIRMED WORKING
Chromium has made substantial changes the way libwidevine (and a few major things around DRM) are loaded/used/etc. They have also made changes to the setting and reading of the user-agent propagation. For some time (~2 months or so) — the combination of this badly broke Netflix. It seems they have undone the lib loading in the last couple of versions, and user @Spartacuss discovered the user-agent fix.