Facebook has now rolled out their event prioritization. This will auto-prioritize based on previous campaign historical the vents Facebook deems most important to your ad account. This can be manually overwritten based on the preference by advertiser. We would suggest looking at and doing so in light of the upcoming iOS14 changes.
Facebook’s Aggregated Event Measurement is a protocol that allows for measurement of web events from iOS 14 users. It limits domains to 8 conversion events that can be used for campaign optimization. The solution is analogous to Apple’s Private Click Measurement, but is designed to solve for key advertiser use cases not addressed by Apple’s proposal. Aggregated Event Measurement will continue to evolve with upcoming browser changes to help our advertisers support consumer privacy. The configuration and account limits imposed by Aggregated Event Measurement won’t apply until Apple begins to enforce their iOS 14 changes. Until that time, you can set up your events to use Aggregated Event Measurement and it won’t impact optimization or reporting.
You should be preparing for Apple's App Tracking Transparency prompt.
And begin prioritizing your Events in Facebook Business Manager Event Manager.