Dentists don't use those rubber dams anymore for root canals?

No: As a general rule, anytime one exposes and instruments the pulp of a tooth, it has to be done with a rubber dam. This is to prevent bacteria from entering the root canal; the whole point of the root canal procedure is to eliminate bacteria in the canal.

Standard of Care: All dentists have the legal responsibility to provide care that the profession deems the standard of care. Root canal treatment requires the operator to control the environment in a way that prevents reinfection and provides protection to the patient from instruments or materials falling down their throat. The rubber dam is currently the easiest and best way to uphold this standard of care.

Rubber Dams: Dentists should and do use rubber dams routinely for root canal therapy. Many dentists use it routinely for fillings as well.