It is hard to say when post-Left anarchist thought "first emerged" because you can find thought that expresses a similar mentality and position to the post-Left anarchist critique even before the critique was actually developed and referred to as such. For example look at the writings of Renzo Novatore or Bruno Filippi. Plus the post-Left critique is not a new social theory but rather a critique of anarchists' relation to the Left and the contradiction that a synthesis of Leftism and anarchy creates in regards to anarchist "principles". For example the anarchist principle of autonomy (the basic unit of which is the individual) is compromised via Left-anarchist acceptance of collectivist ideology and democratic forms of rule. Direct action is contradicted through the institutionalization of activity and organizational forms which include representation and delegation.
I believe the critique first came to be in the 80's through the publication AJODA. Some influential writers include: Jason McQuinn, Wolfi Landstreicher, Jean Weir, sasha k, Lawrence Jarach, Max Stirner, the aforementioned early 20th century Italian anarchists, Raoul Vaneigem, Guy Debord, Alfredo Bonanno and Bob Black. This is a rather short list considering how many different writers have influenced the post-Left critique. Post-Left positions differ from anarchist to anarchist. Some may get more out of Jason's position than Lawrence Jarach's, some may like Max Stirner while others may not, some may take more from the Situationists than others etc. etc. So while this list is my idea of who some influential post-Left anarchists are and I imagine many post-Left anarchists agreeing with me, there is no singular post-Left party line and some may disagree with this or that author I listed.