It has nothing to do with VATUSA, per se. The standard VATSIM flight plan form is still the legacy FAA flight plan form. VATSIM has not yet published guidance on when that will be changing. It's not whether VATUSA condones it or not, it's about the fact that VATSIM doesn't require it. You can put all the ICAO information you want into the comments section, but there is no requirement for controllers to look at your flight plan comments, so all of your ICAO information will likely go unviewed and disregarded until such time as VATSIM changes over to requiring the ICAO flight plan form.