Libical implements the RFCS:

• RFC5545 Internet Calendaring and Scheduling Core Object Specification (iCalendar)
• RFC5546 iCalendar Transport-Independent Interoperability Protocol (iTIP)
• RFC7529 Non-Gregorian Recurrence Rules in the Internet Calendaring and Scheduling Core Object Specification (iCalendar)

as well as iCalendar extensions described in:

• RFC7986 New Properties for iCalendar
• RFC9073 Event Publishing Extensions to iCalendar
• RFC9074 "VALARM" Extensions for iCalendar
• RFC9253 Support for iCalendar Relationships

Plus:

• RFC6047 iCalendar Message-Based Interoperability Protocol (iMIP)
• RFC6638 Scheduling Extensions to CalDAV