Do others think we need this tag?
For that questions I normally ask myself "Myself, what is x509?" and myself answers with a wikipedia dump:
In cryptography, X.509 is an ITU-T standard for a public key infrastructure (PKI) and Privilege Management Infrastructure (PMI). X.509 specifies, amongst other things, standard formats for public key certificates, certificate revocation lists, attribute certificates, and a certification path validation algorithm.
Then, I look another posts/tags that already has what x509 means: "cetificates", "PKI", "PMI", "Privilege Management Infrastructure", "public key infrastructure", other related terms/tags. Now, is the proportion of related terms big enough that it will make it difficult for answerers find the questions they want to answer? No? Then we do not need a tag.
Remember that tags aren't there for the users asking questions but for the users to find questions they want/could answer ;). I do not think we have an important proportion of users that would like to answer those kinds of questions, so NO, we do not need that tag.