Judging by the article below Event Calendar was available before the release of v1909.
But it looks like it may be an app that is only available if you have certain programs installed, and accounts setup.
"Designed for both work and home, these apps help you communicate quickly and focus on what’s important across all your accounts.
Supports Office 365, Exchange, Outlook.com, Gmail, Yahoo! and other popular accounts."
https://www.tenforums.com/tutorials/137149-create-new-event-calendar-app-windows-10-a.html