This is really bad news for the sidecar boys, almost all of who use Avon racing car tyres as far as I am aware. Last ones my mate got were £1,000 for a set of three though!
Pirelli do manufacture an alternative but have refused to supply anyone who (might) do the Isle of Man TT.
Off topic, but the TT organisers have had to change the regulations for the Superstock class, as Dunlop won't supply a treaded tyre, so they're having to allow slicks.