BozLotyer the outcome of a device being unsupported is that it no longer receive OS and security updates from the OEM. Thus, they are increasingly at risk for compromise and instability.
Some other Android projects continue to partially support outdated devices. This support is only partial because they have no way to provide proper, complete updates. They can and do provide some of the newer OS features and security patches from AOSP. However, they cannot provide the proprietary (e.g., firmware) security patches that vendors and OEMs no longer offer. In other words, they can typically port the AOSP stuff but they cannot port the no longer provided, non-existent OEM-specific code.
A recent, notable example is the modem RCE affecting Exynos chipsets and some Pixel models. Unsupported devices will not be receiving a patch for this from OEMs. Other projects may provide the AOSP updates from March 2023 but they won't be able to provide the OEM-specific firmware patch. So, you may receive a false sense of security, "I have the March 2023 updates" when the reality is that you only have a partial March 2023 update. Then compound this every month that the device is unsupported and other high risk vulnerabilities are not patched.
I also recommend that you read the project FAQ on legacy devices.