This is actually not the case. The owner is always made aware of whether the system has been opened up to "untrusted" software or not (they get a warning at boot if it has been), but an "untrusted" system can still be OTA-updated if the original OS image has been preserved as-is. This is often combined with some amount of custom modification by separately installing an "overlay"-based solution such as Magisk. But a full-custom ROM cannot be updated in this way, because the manufacturer's OTA update is monolithic and effectively replaces the original stock ROM!