How are artificial constructions such as l33t classified with regards to English?

Solution 1:

It's a kind of argot, jargon, or insider's lingo.

argot — An argot is a secret language used by various groups — e.g. schoolmates, outlaws, colleagues, among many others — to prevent outsiders from understanding their conversations. The term argot is also used to refer to the informal specialized vocabulary from a particular field of study, occupation, or hobby, in which sense it overlaps with jargon.

jargon — specialist language: language that is used by a group, profession, or culture, especially when the words and phrases are not understood or used by other people

lingo — set of specialized terms: a specialized set of terms requiring to be learned like a language

Solution 2:

I'd just say l33t is an informal dialectal transcription format, not dissimilar to txtese, chatspeak, etc.

Solution 3:

It should be appropriately grouped in with simplistic encryption methodologies, along the same lines as Pig Latin. The wording is rarely dissimilar to standard English, albeit poorly written.