No, it is not the current norm. It was back in the 80's to mid 90's but was just a token. Supplemental doesn't like to supply housing - are you sure you won't be holding the lease too? If so, better to find housing on your own and take their housing stipend. You will probably do better financially, but it is a bit more risk to you and certainly more hassle. If you don't want the hassle and risk, go with another agency that houses most of their travelers. Yes, you will probably take home less, but it may be worth it to you. Everyone has their comfort zone.
The reason you may get paid less with supplied housing is that it is indeed a huge hassle. For an agency to locate housing they've never seen, pay a large deposit, arrange furniture rental, and get the utilities turned on probably takes more hours than anything else that is involved in your travel contract. On top of that, the industrywide rate for travel contracts that fail to complete is 10 percent. Think about that number, it is really important to both travelers and agencies. Agencies take a large risk supplying housing to a new-to-them traveler - it could cost them upwards of 6K if you get fired in the first week. So they can pay more if you are willing to take some of that risk off their hands.