This is over indexing on semantics. The feature name is now Tasks, but the functionality is exactly the same and is now better for Tasks users.

Right, but I created a "Reminder" last year and when I go looking for it now, I'll be looking for something called a "Reminder" not a "Task"

I get that sometimes decisions have to be made about whether to keep or abandon design and UI and nomenclature decisions that are now regretted for whatever reason, but bear in mind that you are creating immense frustration for current users when you do so.

As the designer or manager or developer of an application, you are immersed in it and every change makes perfect sense to you. But your users may use it infrequently and all your changes are just seen as pointless, frustrating churn.

Have you seen this? https://xkcd.com/1172/

It's funny because it's (almost) true.

Is a reminder usually reminding you to complete a task?

What you're calling reminders are just calendar events.

A birthday is a repeating calendar event. You're free to create a calendar event with 0:00 duration in Google Calendar, it works fine. You can also create all-day events that show up at the top and don't make the schedule harder to read.

And also, presumably, you want to be reminded of a thing for a reason. What's the reason you want to be reminded? That reason is the task.

This describes more or less how I use calendar reminders. Typically, if I see a trailer for a film that looks good, I make a reminder several months ahead to check it out. I don't think of these as 'tasks to be completed', but more like 'notes to my future self'.