The interface that you see when editing reminders may appear different based on when the reminder rule itself was created. This depends on whether the rule was created before or after January 17th, 2019.
Note that reminder rules created before the above date will not be converted from the discontinued interface to the new interface. You'll need to delete these rules and recreate them in the new interface if you'd want to create a conditional reminder.
Visual Workflow Interface With Conditional Reminders
Reminders created after January 17th take advantage of the updated visual workflow interface. Here you can make people aware of key dates and times—and even add conditional filters so that only specific rows that meet your desired criteria are included in the alert.
Details on this are available in Automate Your Work With Alerts and Actions.
Reminders created before January 17th have a popup-style (also called a modal) interface. Here you can choose who you want to make aware of key dates. If you need to set a specific time of day for your alerts to be sent out or add conditions to your alert so that only certain rows are sent out, you'll need to create a new reminder rule. (Details on creating new reminder rules is available in Automate Your Work With Alerts and Actions.)