How indicative of maneaverability are the max G numbers of fighter
aircraft?

Somewhat. Of course the A-4 Superfox (A-4F with J-52P408 and stripped of
humpback et al) was an aluminum assassin in the adversary role and rarely
exceeded 6G.

Also, most new aircraft have reported max of 9Gs. Why are they all
coming out at this same number?

Structural design starts getting to be a small problem above 9G (which
implies 13.5G prior to overload), but 9G is about it from the operator
standpoint. You can endure more for a short while, but not while performing
actions other than enduring the G. Even the fittest of the Viper drivers
don't pull max G for long periods.

R/ John