I read a paper from 2013 (Androulaki E., Karame G.O., Roeschlin M., Scherer T., Capkun S. (2013) Evaluating User Privacy in Bitcoin. In: Sadeghi AR. (eds) Financial Cryptography and Data Security. FC 2013. Lecture Notes in Computer Science, vol 7859. Springer, Berlin, Heidelberg) that, regarding transaction input, "currently, Bitcoin clients do not provide support for different users to participate in a single transaction; to achieve this, users would have to modify the Bitcoin client implementation themselves". What about now?
Also am I correct that most of the transaction nowadays have still only one user as input?
Sources (to ex. papers) would be nice.