I was curious as to the current state of each game in terms of pokemon ratios, on their own, if you were to play them now after everything went down. For the sake of me not wanting to cross reference, I'm just lumping version exclusives together & assuming you cant access the mythicals. Also ignoring the whole thing of "you only really get one starter so and there's fossils..." so just pretend the pairs are lumped together somehow. I'll make a note if there's anything especially notable.
GEN 2
GSC: 232/251
An excellent start! The only ones that absolutely require gen 1 games are the starters, the gen 1 legends, celebi and kabuto/omanyte lines. I think Crystal might have a few more "version exclusive" issues but they're negligible at this point.
GEN 3
RS: 200/386
oofa doofa. We all know how this went down, though. Vast majority of these are gen 1s followed by gen 2s which were split across 5 games. I would aruge that gen 3 is worse for this than gen 7 is/will be solely because its split across up to 5 different games to get the full set. what nonsense.
FRLG: 208/386
Surprisingly more than RS! But barely. FRLG had a bunch of events just never released, either, so this number was barely higher during its spported lifetime. You can not under any circumstances evolve Eevee into Umbreon or Espeon while in FRLG. You MUST trade them to RSE to evolve. The only RSE pokemon you can get are Azuril & Wynaut (& Deoxys through the event ticket). The legendary beasts are only available in these two games without Colosseum and you only get one so that's a little extra annoyance but I'm ignoring it for the totals like the starters.
Emerald: 225/386
I'm separated Emerald out because it added some new safari areas (and a few misc post game things). As such I subtracted the 7 RS exclusives. I'm not counting the Johto starters you can get because that requirse completing the Hoenn Dex which is impossible if you're just on its own.
GEN 4
DP: 401 (381)/493
Pretty good! Note that I'm discounting the dongle method since that requires a specific DS model AND a gen 3 (notably al of them game, RSE give 6 each, FRLG give TEN each). Tangela & Tropius are the weirdos who you can get in any way outside pal park and by extension that means no Tangrowth; you also cant get Regigigas since it requires the Regi from RSE. Still, Swarms and PokeRadar really pulling their weight here! The parenthesis number is because serebii's unobtainable list (which forgets Tangrowth... and counts the new mythicals) listed the per-game number off hand. Just there for reference!
Platinum: 397/493
It expanded the normal pokedex, made some of the gba inserts available normally, changed up the unavailables that were available earlier. Lower than DP as a whole, but more than each one separately. A quick note that Rotom's formes are also completely unavailable and have been for a long, long time since the key event was only ran like twice.
HGSS: 451/493
Hot diggity! The only notables missing are gen 4 starters, and all non gen 1/2 legendaries and mythicals. Except wait, no because a large swath of these (that i did not even remotely want to count out) are only available through the pokewalker and the safari. BEcause to get every possible pokemon out of the game(s) you NEED to spend HUNDREDS OF DAYS that you can't cheat at leveling up specific blocks in the safari in certain positions. So it's there, I guess, but how many of those will you actually see? Subtract ~21 if you got HGSS but without the pokewalker. Magnezone, Probopass, Lefeaon & Glaceon are completely unavailable for no goood reason. also do you like unown no reasons just thought id ask
GEN 5
BW1: 473/649
BUT WAIT NO
Because White 1 specifically has the atrocious White Forest Mechanic which can give up to 97 pokemon but this requires speed running the game multiple times to POSSIBLY get like 12 families and it is near impossible to game this system to get more pokemon out of it. It's bad. Everything is bad. I am not counting it
376/649
The dream world ceasing to exist hit this game hard. Otherwise pretty much every normal pokemon was available. Note that Zorua & Zoroark aren't available because they require one of the 4 Zoroark movie events from Gen 4 to get or someone to breed them for you. Also the Stunky & Glameow lines have not been available since DP.
BW2: 483/649
A significant step up considering I didnt have to cut out almost 100 pokemon. The absurd Yancy sidequest lets you get a ton of pokemon which always helps. Do note however, that even between the 2 sets of gen 5 games, there's still a random assortment of normal pokemon otherwise unavailable. Did you know that Pikachu's entire family was never in the wild in either games? It wasn't even made available through the dreamworld, it was only given away via wi-fi events. You're given N's Zorua though, so you at least get that. A sad note: Zen Mode Darmanitan is only available if you Memory Link with BW1. BW2 also had access to the (still available) Dream Radar, which I wasn't counting here, but let you get a few odds and ends and assorted legendaries.
GEN 6
XY: 454/721
Just excise that friend safari entirely its a far off mystery zone where you're trying to complete the pokedex without wifi and I doubt you know a hundred people to fill that friend list locally. Lots of random pokemon completely unavailable even with friend safari, incidentally. Still, over half, pretty solid.
ORAS: 479/721
Despite the tiny dex offering in the main game, it really opens itself up with all those islands and a huuuuge influx of legendary pokemon and starter pokemon from other regions. Its still only over half, but it was designed as a compliment to XY. Also the first games since HGSS that lets you GET UNOWN. Unown was NEVER available in the dream world and NEVER given out as an event and NEVER available in the friend safari; if you wanted to complete it in gen 5 you needed poketransfer and if you watned to get it in XY (before ORAS were out) you needed to poketransfer to gen 5 then pokebank to gen 6. Absolutely absurd.
and thus...
SM: 384/802
Not even half, which is genuinely impressive on multiple levels. What you see in the dex is what you get + the 84 island scan pokemon. Its still a sizable amount of pokemon, but its a really baffling decision to not even at least include half of them. There's not even a dream world or anything. "Stars" or w/e is next will be interesting to see if they're going to continue this course or up the count by a sizable margin. Maybe this will just be their weird plan moving forward...? It is pretty significant that they shifted the national dex out of the game entirely and don't even acknowledge that you might have every pokemon outside the alola dex.