Currently, Calendar imports do not support "Events based on recurrence rules that are not supported by Proton Calendar". The current Calendar UI supports a very limited set of recurrence rules meaning imports are also limited. This significantly impacts the ability to create and/or import calendar events that use somewhat common recurrence rules not currently supported by Proton. In UserVoice, there are a number of suggestions to support various combinations of additional recurrence rules. My recommendation is to "consolidate" these into this (???) recommendation for supporting all recurrence rules defined in the iCalendar specification (RFC 5545) (see https://icalendar.org/iCalendar-RFC-5545/3-8-5-recurrence-component-properties.html).

This recommendation could be implemented in a phased approach where imports could be implemented first to provide the desired capabilities until the UI implementations for adding/editing events can be accomplished. For this to work, I know the Calendar display would need to be modified to display these imported recurrence rules. To minimize that effort, a human-friendly text representation could easily be generated from the recurrence rule for display in the calendar. (NOTE: See an implementation example in https://www.npmjs.com/package/rrule for the 'toText()' method.)

For the benefit of other users/voters, the recurrence properties specified in an iCalendar (RFC 5545) event include:
* Recurrence Rule (RRULE) - The recurrence rule is used in computing the recurrence set. The current set of Proton supported recurrence rules are included in RRULE. (See the RRULE Tool at https://icalendar.org/rrule-tool.html for the RRULE possibilities).
* Recurrence Date-Times (RDATE) - a list of specific date/date-times to include in the recurrence set.
* Exception Date-Times (EXDATE) - a list of specific date/date-times to exclude from the recurrence set.
Even the implementation of these recurrence properties could be implemented in a phased approach starting with the RRULE.