http://u.qdnx.org/file/rozzin-20160908T205005-jqxcr2q.html
For a while #GoogleTalk was still available via #Google's webUIs, but then they switched the back end on those to Hangouts for everyone and removed the option for which back-end was in use. And for a while they displayed prominent notices about what was going on, but then eventually they stopped doing that. So if you went some period of time without using #GMail for example, you might have missed all of the banners.