I'm not aware that either the Angeles or San Bernardino Nat'l Forests are under a total closure order. If they are, that's news to me. There are some specific areas (ranger stations, camp grounds, recreation sites, visitor centers, etc) that are closed, for example Chantry Flats and Vivian Creek/Big Falls are both closed.
If a specific area within a forest is closed, they have to post it as such.
I haven't heard anything about Icehouse Canyon specifically. Maybe it would be good to check on the
San Gabriels Forum? If you arrive and the lot is closed, you could always park on the road. As long as you're early, there should be parking. Or you could park in town if you needed to disguise what you're up to.
HJ