WhatonEarth The sensors permission is not a permission that's available for other Android OSes. As a result, it's not a permission that can be treated the same as other permissions. Apps have no way of requesting the sensors permission like they can other permissions in Android, like access to the camera.
I'd suggest reading the short section on the sensors permission on the website here: https://grapheneos.org/features#sensors-permission-toggle. One part that's relevant to your question basically states that the sensors permission is granted by default to avoid breaking things.
WhatonEarth Is there really no way to disable this?
WhatonEarth Why would developers force this upon users?
WhatonEarth Furthermore, the "Notifications" permission can't be disabled from the SIM Toolkit.
It's my understanding that some apps' permissions cannot be changed because of restrictions set by upstream AOSP, others are set by GrapheneOS. In both cases, the restriction to change permissions has been set because changing permissions breaks things, sometimes in unexpected ways. This happens more often than you might think.
WhatonEarth How do I physically remove or disable the mysterious "sensors" (we're never told what they sense or how many there are) from the phone without ruining it?
This isn't a thing that we would ever suggest. The sensors permission blocks apps' access to sensors data. It makes more sense to use the sensors permission than to risk damaging or destroying your phone.
That said, the link I shared above lists the following sensors: accelerometer, gyroscope, compass, barometer, thermometer. I don't know if that's all of them. You'd have to check the specs about your device to get a better idea of what's all there...
WhatonEarth when I go into App info>Sim Toolkit>Permissions and select "See all apps with this permission", "SIM Toolkit" is nowhere to be found.
If I'm seeing the same screen as you, you should be able to tap the 3 dots in the upper right-hand corner, then tap show system to see all system apps.
WhatonEarth Plus, there are over 100 apps on that list, yet when I go into "All apps" in settings only 16 are shown. Has anyone else noticed these secrets?
All apps also has the same 3 dots > show system menu.
WhatonEarth In addition, at the bottom of App permissions>SIM Toolkit, it says "To protect your data, permissions for this app will be removed if the app is unused for a few months." Something's fishy about this one way or another: whether they're lying about the permissions being removed or they're telling the truth and your only option for removing the permissions is... ... ... waiting. This seems very bizarre. I'm sure some people understand it all, but I sure don't.
That message is referring to a feature in Android where if you don't use an app for a long time, many of the apps permissions will be removed. I'm not sure of the specifics of what else happens, but this has happened on my phone multiple times.
I doubt that SIM Toolkit's permissions will be removed since it's a system app and has those permissions for a reason.