Google and Outlook cover a lot of calendars, but not the Apple one on your phone or the Fastmail account you moved to. Both speak CalDAV, an open standard for calendar access, and Keeper.sh now connects to anything that speaks it.
Connecting takes your address and an app password rather than your real one — a separate credential you generate for a single application and can revoke on its own, without changing the password you sign in with. The connect dialog links to the page where iCloud and Fastmail issue them, because both bury it somewhere different.
Once connected, a CalDAV calendar behaves like any other source. It is read on a schedule, its busy time reaches your other calendars, and it can be a destination as well as a source, which means a self-hosted or standards-compliant server works without Keeper.sh needing to know the product by name.
Read more about the calendars Keeper.sh works with.