I was curious to see that in recent releases the humans are limited to a bronze age culture. Is this a deliberate timeline repositioning or a bug?
It's always been the case that steel was the dwarf-only metal. Humans have had to make do with whatever was below steel. It's just that Bronze is now better than Iron, so therefore they go for Bronze. It's actually realistic - pure iron is worthless compared to bronze and what we call the Iron Age is actually the "Steel Age".
Actually it's due to the Humans not having anything other than Brass, Bronze, Electrum, Billon, all 3 Pewter types, Black Bronze, Sterling Silver & Rose Gold available because nothing else is specified in the raws for them. They don't even have access to Copper or Tin to make the Bronze they use in everything. You can see this if you modify your raws to add the Human merchant guild rep back in as you can't request any metals other than those 11 from the Human merchants.
As Bronze is the only one of those 11 alloys which can be used for weapons & armour, they have no other choice but to make their stuff out of it. If you add access to Copper & Iron ores for the Humans, they then have the possibility of using those for weapons & armour.
As Bronze is better in most of the non-broken combat methods at present it is technically "better" than Iron right now however as it stands, the Humans having some magic recipe for making Bronze without access to Copper & Tin is just comical.