'plaintiff' is still being used occasionally in England, as it used to be the normal term for the claimant. However, since the Lord Woolf Reforms came into effect in April 1999, the official term is now no longer 'plaintiff' but 'claimant'. This terminology is certainly being embraced by legal practitioners and taught at law schools in England. More on the Lord Woolf Reforms can be found here: http://www.ladas.com/BULLETINS/1999/0899Bulle...
- the third paragraph for example highlights some of the changes in terminology, including plaintiff --> claimant.
My Blackstone's Civil Practice has a table at chapter 100 (table 100.1) which lists the old and new terms, including Old term: plaintiff, New term: claimant.
I certainly would not suggest flagging it as obs. (yet) as the term is still used in older documents, but for the last 6 years, at least in England and Wales, it has been replaced by 'claimant', hence my suggestion of (veraltend).
I can't speak for any other English speaking countries, so I don't know whether it is still used there.