Only works if you haven't updated it since the February 2021 update. You also would lose the actual built in Chromecast feature.
Cool but I don't really see why you would do it on this device when you can just flash android to a raspberry pi or something and have basically the same thing at that point.
I have one of these sitting new in box unopened. If I have time later I will give this a try.
EDIT: To answer what you said, the reason you might want to do this is that this hardware doesn't need to be chromecasted to by a phone or laptop or something. It can have native apps installed directly on it, and be controlled via your normal TV remote control.
Granted I haven't tested this yet to see if it works with the alternative OS.
It would be truly awesome to have a degoogled streaming device that you could sideload apps on, use your regular TV remote with CEC etc. Even better if you could use alternative apps to get ad free YouTube streaming.
In contrast, for the unmodded device, the default GoogleTV setup and UI is horrible. You have to manually remove all the garbage apps you don't want. It still shows you suggested banners of terrible content exclusive to certain apps that you don't even have installed.
If you mean you have an unopened 4k Chromecast chances are you don't have the exploitable firmware. I have one NIB right here and it's dated 2024 mfg so chances of being unpatched are nearly 0.
I have an orange pi running android 12 hooked up to a TV and the lack of native apps for stuff like hulu [tablets not supported.] and I have the navigation bar stuck on the bottom even in full screen mode. I need to figure out a way to hide that. So it's not quite so easy. If Lineage can come with the TV configuration working out of the box, then for 50 bucks WITH a remote... its an absolute STEAL.