The [ABM] treaty was signed in Moscow on May 26, 1972 by the President of the United States, Richard Nixon and the General Secretary of the Communist Party of the Soviet Union, Leonid Brezhnev; and ratified by the US Senate on August 3, 1972.
The 1974 Protocol reduced the number of sites to one per party, largely because neither country had developed a second site. The sites were Moscow for the USSR and Grand Forks Air Force Base, North Dakota, since its Safeguard facility was already under construction, for the US.
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Bush exited the treaty around 2002, but of course the ABM systems are still there. It is indeed true that the first ABM around Moscow was built with Galosh, but it was upgraded several times, and today it is A-135.
I don't know, of course, how those S-400/500 fit into this system, but one thing is obvious - Pantsir is so cheap and apparently sufficiently effective that they decided to sprinkle a few of those around just for good measure. Add high speed data communication and some decent computers to coordinate everything, and with so many radars you can see everything worth seeing, and with so many different missiles you have several options of intercept. As I said, that could be important if there are many warheads coming your way. A 300 miles, 5 min. reload time missile is of no use if the threat is 10 miles away but you need to fire one missile every 30 seconds.
They also have deployed Tor M-1’s around Moscow as an intermediate measure.
BTW, they are deploying these things in St. Petersburg. Belarus is supposed to get some for Minsk, too.