Ideally, once the summer 20 has been released the two updates are removed on the critical update section. It might also be possible that in Production, you have activated those Critical updates that is why they are not showing.
Not activating the opt-out option means you are allowing Salesforce to enforce the Guest User Object Permission on your org.
Once the deadline is reached and you haven't activated the Opt-out Critical update, the following permission will be disabled:
-Edit, delete, Modify All Data, and View All Data for custom objects.
-Edit, delete, Modify All Data, and View All Data for the following standard objects: Order, Contract, Survey Response, ProfileSkillUser, and ProfileSkillEndorsement.
Activating the critical update keeps the following permissions unchanged. If they are enabled, they remain enabled, and if they are disabled, they remain disabled.
As for the notification for critical updates. I have found an Idea regarding this. Here is the link:
Send Administrators Email Alerts of Critical Updates
What I can suggest for now is to upvote the idea. If it is feasible and noticed by our Product Manager, it can be included in the next release.
If you want to opt-out, has to be should be a manual intervention.
I hope this answers your query.