The ICAO flight plan format became mandatory for all IFR and VFR flight plans on August 27, 2019.
FAA Notice NOTC9616 provides the details. You can find addtional guidance here at BruceAir at ICAO Flight Plan Equipment Codes for Aircraft with IFR GPS and at the FSS 1800wxbrief.com website.
Most pilots today file flight plans via apps such as ForeFlight, Garmin Pilot, FlyQ, or WingX. If you enter the equipment codes for the aircraft you fly into your cockpit app or web-based service, filing ICAO is trivial.
Learning how to complete a paper flight plan form isn’t necessary, and the apps ensure compliance with ICAO and FAA standards. The apps also typically provide help with understanding the ICAO codes used for most GA aircraft and operations.