As far as i can remember we had on vatsim some real restrictions, that makes two main rules:
1. no speed restrictions above sea (transatlantic, transpacific).
2. optional no speed restrictions in two cases:
a. if atc agree to no speed restriction for airplane - there is no speed restriction then, but only in that single flight in that exact controlled space
b. if there is no atc online in specific area - there is no speed restriction, but watch for other traffic and use your head.
In any other case (over ground and/or in controlled space we have only subsonic speed alowed.
And that is only for airplanes available in scheduled/charter flights, does not cound xxx of liveries of other planes, like private Cessnas or other stuff that fly under ACH livery not with company flight...