i think this question will always be impossible to answer in a way that satisfies everyone bc some people look at it as how old the era is while others look at specific changes to the cards themselves.
i subscribe to the latter
we can all agree base-neo at the very least is vintage.
- for japan, this is clearly distinguished by the back of the card being different from everything after
- for eng, this would extend to Legendary Collection (the WotC era)
however e-series was the first big change for jp
regardless, i think we can all agree that base-e/legendary collection is one era (gen 1-2)
next would be gen 3 which introduced ultra rares (exes) as well as a change in set order that lasted through HGSS internationally
if you want to simplify the eras, this would also be tied with gen 1-2 IMO bc this is also the last years of set holos being the cosmos foil
Note: this era is also distinguished from 1-2 as "Retro" for TCG players, whereas 1-2 are considered "Legacy" formats
next would be the biggest change in the TCG globally since its conception, BW era
along with this change for international set structure, it also introduced the modern JP rarity symbol system
Note: BW is also the earliest "Expanded" format goes for the sanctioned TCG format outside of Standard
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XY added onto the BW system, but was not an overhaul (stuff like introducing the double rare - RR and the introduction of Special Arts in sets as an even fancier Full Art for example)
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SM did introduce quite a few things, but really it just added a bunch of new rarities such as rainbow cards, full art gold cards, character arts, shiny rarities, and full art items/stadiums.
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SwSh was a continuation of SM in just about everyway. IMO, these two gens (like BW/XY) should always be lumped together bc aesthetically they are nearly identical
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SV is the next biggest overhaul to international set since BW (so if we’re counting, this would be the 3rd major era in the grand scheme of things) where it not only homogenized everything to have silver borders, but also adopted the JP rarity/set order system for secret rares as well as adopting things like double rares (double star).
so, with all that said, i think BW on is “modern”. everything before and after is up for discussion with how picky you want to be.
i do like the idea of “ultra modern” (although i would call it something more unique, like contemporary aka current) starting with SM because the cards then are really quite similar to the cards now in almost every way aside from the borders (for non-TPC printed regions)
if you want to know where i grabbed these screenshots from, you can check out the articles i made with PFM regarding Set Order and Rarity overhauls throughout the history of the TCG