(%cat)''s''

This "s" bit on the end exists on a number of messages to me translated, such as Interwiki-adding-modifying (“Bot: Adding %(adding)s; modifying %(modifying)s”).

Does it indicate that the category should be in plural form, or is it part of the syntax?

AFAIK, the %(variablename)s are variables by the software that are replaced then, like $1, $2, ... in MediaWiki. The "s" in the end probably means that it will be replaced with a string (d means digit). A plural function should probably be added there.