And why can't humans enjoy cheese and socks?
Will this ability to generate new object types mid-game allow us to potentially add new modded objects mid-game as well?
It might be a start, but I doubt it. There is probably more that needs to be done for that to work, such as doing a diff on the save raws and the data structures of the save.
This may have been answered already but in the next version with the addition of history continuing to be generated post world gen and the possibility to retire fortresses will the embark options change based on what we do in our own forts if we retire them? ie if we tame elephants long/well enough or embark in an evil biome and get a hold of some silver barbs will those then become available from the embark screen in future forts?
If no, is this a planned for the future?
There was a bit of this further upthread with regards to domestication. Either way, what your fortress is doing is supposed to impact the world. In some future update, if you manage to domesticate elephants (it's currently not possible to reach domestic status for undomesticated animals), then you should eventually be able to get elephants through caravans, assuming your civ can get access to enough elephants to domesticate and trade away. Similarly for sliver barbs - if you put enough of them into circulation, you might get some sliver barb products into the world, which may then reach your future forts. If you send out seeds, that might end up granting the civs around the world the ability to plant sliver barbs.
Is the reverse possible? Deleting all objects of some type?
The dark god of elves bans the existence of axes and anvils. It is up to our young dwarf hero, champion of Armok to restore unbalance to the universe...
That seems really doubtful. Deleting object types holds pitfalls that adding them doesn't. And in your example, you certainly don't want to delete anvils or axes - you want to destroy the existing ones, then edit the civilizations to remove the ability to make axes and anvils from them, and then set up some events that restore that ability to those civs. Deleting the item types in question would be counterproductive in this case. I suppose it could make some sense, for example, for curing all vampirism, but even then, I'd keep the vampire curse objects around for reference at least.