I thank all the fellow users who contributed to this thread with constructive information. To answer some of the questions above: my build number is 2026011000, the device is Pixel 8a, the region is Italy, system language appears as "English (Italy)"; the menu page in question is titled "Public warnings", the top toggle is called "Allow alerts", above it appears the text "Critical messages may come through, regardless of user settings". Down the page there is a section "Alerts" (Presidential alerts, IT-Alerts, Test messages, Emergency alert history) and the "Alert preferences" (Vibration, Alert reminder, Always alert at full volume). If this was just a piece of leftover text and the main toggle did actually work it would be great. If any dev could confirm that the intended behaviour of the OS is to actually block all warnings and do not display any pop-up text, I would be very satisfied.
If that is not the case, though, let me reiterate that my question is just: Why this notification cannot be completely disabled, while detailed amount of configuration is allowed for all other notifications? I have read several instances of the non-answers that I had already outlined in my first message, such as "it is harmless", "don't worry about it", "code it yourself". To those fellow users: You are of course not required to help me clarify this issue. If you take the time to reply, a constructive reply that actually addresses the point I raise would be more reasonable. Why does it bother you that I ask this question? You are most welcome to move one and do not answer me, of course.
In particular, concerning the "it is harmless" line of stonewalling, let me be more blunt: You know what is also harmless? Personalized ads. I challenge you to show me examples of someone that was "harmed" by a personalized ad. I do not have evidence that any of the big phone brands collecting user data has ever "harmed" any user. Evidently, this would negate the whole premise of a privacy-focused project like GrapheneOS. Actually, I think that a third party being allowed to command the appearance of graphics on a device is much less "harmless" than any of the privacy-violating behaviours that GrapheneOS is built to prevent.
I thank anyone who demonstrated interest in this topic, and I will read with great interest any further contribution that clarifies the issue.