Apple / .ICS calendar integrations can be enabled by integrating an 3rd party calendar with a .ICS link.
FROM THE WEB APP
Step 1: Go to Settings > Calendar Integrations
Step 2: Click "Add" and select "From .ICS link..."
Step 3: Paste in the .ICS link from the calendar you wish to integrate.
Step 4: Save. (It might take up to 10 minutes for your calendar to finish syncing).
FROM THE MOBILE APP
Step 1: Go to Settings > Integrations > External Calendars
Step 2: Tap 'Add' and connect a 3rd party calendar.
Step 3: Select "From .ics link ..."
Step 4: Save (it might take up to 10 minutes to sync your .ics calendar).