This topic came up in a meeting we had yesterday. At first we were thinking this request was asking for a web-based calendar that people could embed or iframe onto their site. Unfortunately this would be really complicated for us to do right. After shooting this idea down we realized that the public "webcal" feature we have should satisfy the request for a publicly accessable ICS file containing all the events.
-Go to your calendar section, click on the calendar name to bring up settings
-Enable "Allow others to subscribe to this calendar"
-Copy the URL and change the webcal:// to https://
-Insert this url into plugins that accept ICS, you can also put the url in your browser and download the ics file to inspect yourself.
I tested this with some clients real quick and it seems like this would work, however I did not try any real-world scenarios like wordpress plugins.
Lemme know if this works out for you, hopefully it does. We did shoot the idea of a js/html/iframe based calendar down, too complicated. However I'll go ahead and mark this thread as "Completed", if you experience issues using this URL we can look a bit more into it.