I'm aware of the cosmetic effects, city sounds, appearance etc. and if you start at a later era CIV4EraInfos.xml sets your starting units, gold etc. and the bNoAnimals, bNoBarbUnits bNoBarbCities, but are the other parts of CIV4EraInfos.xml just for games starting in that era? I'm trying to understand what reason one would have for needing multiple eras in a mod if you didn't need to use the sound/graphics changes or intended for people to start at a later point in the tech tree.

You wouldn't, that's pretty much what Eras are for.

The first two eras are Early for unit art purposes, the next two are Middle, and all after that are Late. (Settable in unit infos and unit art infos--I wish I knew how to change this to have more categories or a different grouping). Music changes (mostly leaderhead infos and era infos). City and improvement art change (the L system files). Later starts award different buildings and units building infos "start era" tags and era infos).

It seems like the AI has different strategy depending on era, but that's probably just a secondary result of other conditions, not pegged to a specific era label. From what I've seen of the SDK (all of it, having just spent an entire day attempting to merge 3.19 file by file with Revolutions) and understood (a little of it) there's nothing that specifically references certain eras. Specific stuff in Python could probably be set to be era dependent, such as events.

If you don't want different eras, you shouldn't erase them, there are too many references to other files.
Just set the era to one special for all the technologies. It's dirty, but it's quick.

Is there a way to change the flag's look for the future era? I want to use the flag/hologram from the Final Frontier mod in the future era in my next mod. Is this possible?

The hologram effect is iirc a global thing :(.

The hologram effect is iirc a global thing :(.

Yeah but is there any way to make it a non-global thing? Where do I find the code that dictates this stuff?