GrapheneOS still intends to support all the supported devices until EOL. The sideloading change doesn’t affect them. It won’t apply to GrapheneOS. It only applies to certified OSes and GrapheneOS is not certified because it doesn’t license Google Mobile Services. As per the rip out of the device trees for Pixels, that just makes Pixels like other phones. GrapheneOS has been able to expand it’s automation to build that device support themselves. For new devices, making the support will take longer than it did in the past though, but they will still support those Pixels, as long as they meet the hardware requirements and still allow third-party OS support with all security features intact. Besides that GrapheneOS is actively talking with a major Android OEM right now in order to help them reach the security requirements for a subset of their future devices. They are very optimistic about that.

Android is Linux of course since the Android kernel is a Linux kernel. I’m aware you are probablly referring to using traditional Linux OSes that are typically used on desktops on mobile phones. That would, however, be a significant regression for security. Android and iOS are both modern mobile OSes with an in-depth security model which includes a mandatory app sandbox with a sane permission model. This is not present on traditional desktop OSes. This is not meant to diss on those OSes, they are just children of their time, they were created much earlier, security practices have evolved. I can see why it would be a fun experience though to tinker with, it would just not be a secure experience and it’s unlikely to get there because the improvements in traditional Linux distros go much slower than they go on Android and Android is already massively ahead.