Comment 300-1200 transition was early-mid-1980s (Score 2) 410
At least for CRT terminals and Unix-capable computers (PDP-11s, Vaxen, and most of the Motorola-based machines), 1200 baud took over from 300 baud pretty quickly. We still had 300 baud paper terminals like the TI Silent 700, and people with Commodore 64s or TRS-80s might be using 300 baud, but otherwise 300 was mostly gone by the mid-80s. I last used a Silent 700 around 1991, dug out from the storage closet at the lab because I was working on a project with a company that wouldn't let us connect to the outside world from their LAN, but their security people were ok with a Silent 700 on their phone network, figuring it couldn't compromise anything. It was an old and clunky way to read mail, but it sort of worked.
There were exceptions - the Visa credit-card protocols ran at 300 baud, because it was much faster to sync up at 300 (3-4 seconds) and send less than 100 characters of data than to spend 45-60 seconds syncing up 1200/9600/14400 and send anything.