There is no dearth of custom ROM choices in the aftermarket development scene, but if there’s one that we could consider to be “the biggest” of them all, then that would be LineageOS. There are so many reasons why the LineageOS project is loved by the community, and it’s not just for all the features that the custom ROM offers on top of vanilla AOSP. Thanks to the Device Support Requirements Charter, if you choose to flash an official build of LineageOS on a supported device, you can be rest assured that your device will be running stable software that is also kept up to date against serious security vulnerabilities — often well beyond the end of official manufacturer support.
Just because your device isn’t listed here does not mean that you can’t get to enjoy LineageOS 18.1 by now. Because of its open source nature, there are a number of unofficial builds for many devices on our forums, many of which will eventually end up becoming official builds as development progresses. What’s more: most of them are, by now, perfectly stable as daily drivers, with the occasional minor quirk.
If your phone no longer receives OEM updates you do want to seriously consider switching to LineageOS.
See LineageOS 18.1 based on Android 11 is here for nearly 60 devices
LineageOS 18.1 builds based on Android 11 are now available for dozens of smartphones. Here's the changelog and supported devices list!