Actually, Halon is still available. Quantities are limited and the prices reflect this, but it is still sold to the public.
On the question of damage to the environment, there is a debate on the legitimacy of this claim. Regardless of the position one takes on this issue, Halon still is one of the most effective extinguishing agents on the market. The major downside to Halon is that it is not effective in windy conditions. When a fire extinguisher is needed under these conditions, your dry chemical extinguishers should be considered.
On the question of the toxicity of Halon, it should first be emphasized that there is no fire extinguishing agent that is totally safe to breath, especially the dry chemical agents. The following is from the H3R website (http://www.h3rcleanagents.com/support_faq_2.htm
Halons are low-toxicity, chemically stable compounds that have been used for fire and explosion protection from early in the last century. Halon has proven to be an extremely effective fire suppressant. Halon is clean (i.e., leaves no residue) and is remarkably safe for human exposure. Halon is a highly effective agent for firefighting in closed passenger carrying areas. Due to its effectiveness and relatively low toxicity, the FAA continues to recommend or require Halon extinguishers for use on commercial aircraft.
Extensive toxicity evaluations have been compiled by nationally recognized United States medical laboratories and institutions on Halon 1301 and Halon 1211. These evaluations have shown that Halon 1301 and Halon 1211 are two of the safest clean extinguishing agents available.
For automotive use, especially in the passenger compartment and under the hood, Halon can be exceptionally effective and, as Larry points out, it will not leave a caustic residue as the dry chemical extinguishers do.