Does anyone actually publish "structured proofs"? In 1995, Leslie Lamport published an essay in the American Mathematical Monthly titled "How to write a proof".  In the essay, Lamport introduced the concept of a structured proof, in which the traditional high-level proof is augmented by a sequence of lower levels.  Each level of proof expands each step of the higher level into substeps.  The amount of detail at the lowest level is rather extreme -- Lamport's proof of the irrationality of the square root of 2 runs to 1.5 pages.

The essay has over 250 citations according to Google Scholar, but I have never seen a proof published in this format.  In a PDF or on paper, the extreme detail could be overwhelming; but I think modern web publishing platforms could accommodate it very well (with hierarchical collapsible subsections for each part of the proof).

In any case, are there examples of proofs published in the format suggested by Lamport?
While most people use more traditional proof styles, the more exhaustive structured proof style that Lamport proposes seems to be adopted, at least in spirit, by machine-assisted proving systems like Coq and Lamport's own TLA+.  By bringing a very non-intuitive player into the loop (the machine), these systems force the mathematician to be much more explicit about every step and assumption, and encourage hierarchical structure by their use of programming-language style syntax.  

These proving systems are typically quite expensive to use, and you generally wouldn't want to put their lengthy proofs into the middle of a paper---attaching as supplementary information is more the way to go.  They are, however, proving to be quite valuable in critical applications where dealing with every possible edge case is important, such as in the design of microprocessors or cryptographic protocols.