I've been taking Bus 40 for years and it definitely used to go to London Bridge and onto Gracechurch Street up until this summer. Perhaps TFL wants more people to use the Northern Line since it's not crowded enough...
This was part of the same major changes to bus-routes as the withdrawal of the RV1. Unfortunately, concern in SE1 about the RV1 seems to have detracted attention from the other changes.
MarcoC: 'Perhaps TFL wants more people to use the Northern Line since it's not crowded enough...' may be right - if you look at the various changes they seem to designed to stop south Londoners taking a direct bus across central London's crowded bridges to places north of the river!
Why the new 40 route should terminate at Clerkenwell Green instead of extending to King's Cross I don't know. Perhaps it's due to TfL's fond belief that once (eventually) Crossrail/Elizabeth Line opens, Farringdon will become a central traffic hub.
What you realise when you look at it is that there were once a large number of cross-town bus routes running from the suburbs on one side through the centre to the suburbs on the other. (Or even further - does anyone remember the old Green Line coach that ran all the way from Dorking to Luton via Victoria?) Gradually, as traffic increased, LT realised it was impossible for these routes to keep to timetables through the congested central area, and cut them short or turned them into two overlapping sections (like 453 and 53).
Their argument now seems to be that with current types of ticket and payment options you can change buses without paying twice. Doesn't help the inconvenience, of course, or the stress on those using major changing points like the Elephant and Castle - confusing unless you're a regular user and know exactly which bus-stop your connecting bus goes from!