effe

I don't understand why you say that effe is used only in chat and sms language. There are many hits of the word on Google books. As for the criteria: if it is attested according to WT:CFI, it can be included. So that includes forms without -n and other colloquial respellings.

You are right. I have never seen it outside chat and sms language. It really looked the same to me as e.g., (non-subjunctive) "lope" and (plural) "bure" for respectively standard "lopen" and "buren". But apparently, effe has been used in texts.