Some template parameters are essentially boolean. These include 'deadurl' and 'nopp', both used by [[Template:cite web]] (and others in the family).
With 'deadurl', "no" is the only real "false" value; everything else is true/default behavior. For 'nopp', if it is set to anything, it suppresses "p." and "pp." before page numbers.
In these examples, often the parameter is just omitted for the default behavior (i.e. instead of deadurl=yes it can just be left out).
I think that is common with such parameters, but there may be examples where it doesn't hold up (e.g. omitted is an undesirable case, and it's better that the parameter explicitly be set true or false).