Wikipedia claims 1200bps was available in the 70s. https://en.m.wikipedia.org/wiki/Modem I did start with a 1200bps modem back in the 80s, even then it was considered quite slow but was enough for email reading and basic bbs tasks. It sucked for any kind of download though.
1200 bps didn't really show up as something people could buy until the early 1980s, however. In the mid 1970s if you had a 1200 bps modem it would be equivalent today to the cost of having a cisco or juniper router with several ports of 100GbE interface in your house. They were in use for university mainframe/minicomputer data links and such.
