I think the problem is with the new deck images. Dek.im must hide the source of codes from searching, while the old version let it.
Nope, it's exactly the same for searching.
Something has been borked with SMF search for a little while now. It won't return more than 1 page of results when there should clearly be more, etc. It started when the forums were updated to the latest 2.0.6, guessing something happened. I'm having trouble tracking down the issue.
The workaround for now is to use Google like this:
site:elementscommunity.org/forum 8pj
That would return all decks with Mark of Entropy. You can of course replace 8pj with a deck code, etc.