The Google map with a London bus route overlayed onto it has disappeared
from the TfL website.

The URL for this was:
http://www.tfl.gov.uk/tfl/gettingaround/maps/buses/

The above now redirects to:
http://www.tfl.gov.uk/tfl/gettingaround/maps/buses/tfl-bus-map/text/

The unofficial 'On A Bus' site has also gone:
http://onabus.com/

I'm guessing this might be because Google has implemented charging and
usage limits for high traffic websites, as described in the blue box
further down on this page:
https://developers.google.com/maps/documentation/javascript/usage

If so, then TfL should really get on the case and roll out a version
that runs on Open Street Map, which they already use for the bus
countdown maps, e.g.:
http://countdown.tfl.gov.uk/#|searchTerm=71234

Indeed, given the advance warning, this could have been done ages ago.

That said, maybe my guess is wrong, because the journey planner on the
new (beta) TfL website uses Google Maps....
http://beta.tfl.gov.uk/plan-a-journey/

Unless TfL have decided that the Google bus maps should be discontinued,
with the expectation that the new journey planner is a good enough
alternative.

A shame if so, as it was a really useful facility which meant you could
easily look up the exact route a bus takes, and see all the bus stops it
serves.