what is the global internet telemetry measurement collective llc

There's another discussion thread on this topic, (link below) with a reply from a Community Specialist, (ie: suggesting how to unsubscribe from the spam calendar)

global internet telemetry measurement col… - Apple Community

All the best :-)

open you Mail application settings (Apple native) and check accounts that are set up - there should be a spam calendar added that you can remove. If you don't have the Mail app installed, download it from App Store; not having it installed does not prevent accounts from being added.