darcs help environment says:
DARCS_PAGER and PAGER:
Darcs will invoke a pager if the output of some command is longer
than 20 lines. Darcs will use the pager specified by $DARCS_PAGER
or $PAGER. If neither are set, `less` will be used.
This should hold for whatsnew, too.
Also, the 'v' key in interactive mode should behave like 'p' (i.e. use
pager) and 'p' can disappear. I know of no use case where you want more
than one page of output to rush through your terminal in one go...