Is there still a rule that Interstates can't be tolled or has that been thrown out the window?
They can be tolled, but then they won't receive federal funding.
----
What about tolling trucks only, like I-95 in Rhode Island recently started doing?
Going by memory here, the tolling option for existing Interstates (not counting the building of additional toll lanes) dates back to TEA-21 initiative that offered up to three
slots for states to do such
under strict conditions regarding where the toll revenue would go (such was why PA's initiative to toll I-80 was shot down more than once by the feds a decade ago).
The three states that initially applied for it never acted/implemented such (i.e. left the offer on the proverbial table).
With regards to RI's truck tolls; the feds likely approved such because the money was going the bridges & overpasses along each tolled facility
and the fact that only trucks (semis) would be tolled, not passenger cars or light trucks. Although, there is at least one lawsuit filed against the State of Rhode Island by trucking groups or companies that
could if successful send RI, and some degree, back to the drawing board.