I have tried microG Services Core v0.2.12.203315 on several different Android devices. I tested original LineageOS for microG ROMs but also LineageOS "N"igthly versions as well as /e/ OS ROMs with subsequently installed microG Services Core v0.2.12.203315.
For all installations without exception the “Exposure Notifications” were set to “Off” by default. Only when an application explicitly asked for “Turn on Exposure Notifications” and confirmed with Tap on the button “Turn on”, the activation took place.
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So if you don’t have a corresponding application on your phone, this function will not be activated and it will remain disabled.I am curious how the better solution will look like …