Rated 4 out of 5 stars

This add-on works exactly as described: extracts embedded ICS file from Outlook/O365 calendar invites, which allows for download and import into other calendar apps (e.g. Apple macOS Calendar). The gotcha is the invite must be sent to a non-MS account for the "get appointment" button to work; if it's sent from MS to another MS account, the button is non-functional. The workaround is to forward (inline) the invite back to yourself, and the button will work on the forwarded message. Kludgy, but at least provides a functional option until the bug can be addressed. Thanks to Leon and contributors.