Originally Posted by
shelbyfv
Yes it was. In addition to modifying as you mentioned, Shimano now has a 34t 11 speed cassette that will fit the 10 speed and lower hubs. There was a thread recently.
The Ultegra R8000 11/34 cassette will fit a 10 speed body. Cassettes with smaller large sprockets will not.
Edco makes an 11 speed monoblock cassette to fit a 10 speed body. As mentioned, there are various hacks such as machining the back of the mounting spider, but that can make spoke clearance dangerously slim.