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LineageOS 21 now officially supports Chromecast with Google TV (4K)
(9to5google.com)
This is amazing. Unfortunately it's only viable for older builds for now:
These instructions will only work on devices that have not yet been updated past the February 2021 OTA.
You can check this in the Settings application under the “System”, “About” tab, labeled “Security Patch Level”.
If your device was updated past this build, it is not vulnerable to the unlock exploit utilized below, and you cannot run LineageOS on that device.
Downgrading to older builds is not a viable option.
If your device is on the February 2021 OTA, or a build that preceeds it, you can proceed.