I don't believe they are on at the moment, no. As you suggest - best to keep an eye on the circuits own website. MSV for one are pretty good updating their covid info.
I got a few in last year during periods of eased restrictions. Was the only events I did do last year! The nature of them mean restrictions only need to ease up a bit before they can go ahead again.
I suppose all of these organizers are allowing people to book as 1) restrictions could be eased at any point and allow them to go ahead so probably no sense cancelling them until the week of, and 2) means they're still getting some cash in to stay afloat, if they cant go ahead now at least you're top on the list for dates later in the year.
I think there is going to be massive demand for track days and other motoring events (e.g. goodwood FoS) later this year. Lots of people carrying bookings over from cancelled events mean there wont be many spaces left and people will generally be desperate to get out of the house!