Hopefully I can clear up any confusion. Its weird but it makes sense that if you buy Shadowkeep (SK) during Season 9 or 10, you get THAT Season's "Battle Pass" (we'll call it BP. Poor terminology calling it Season Pass considering that meant something totally different 10yrs ago). But again it makes sense bc from that Q&A, it almost sounds like SK comes with...a free BP voucher for what ever Season you buy it on, but they don't really concretely mention it that way. Kinda like Gamestop subbing you to GameInformer for "free" but they didnt tell you they were using ur Power Up Reward points. Bungie version: "hey you can get Undying's BP free but we're actually using your "voucher" which can honestly be claimed during any season throughout Year 3"
As for what content do we get when you buy Season 8 (SK)? https://www.youtube.com/watch?v=mEvVKCKdtfw forward to 8:45. Above the word "Calendar" you can a general idea what's gonna be free for New Light players and SK players. I do see what you mean by "What are we paying for" bc it looks like New Light players are gonna get access to the Moon and 2 new Strikes which sounds like that should be SK-specific stuff, but again, they said they wanna make it easy to bring your friends into Destiny, and New Light was gonna be that entry point. Imagine bringing in a friend but you guys can only do stuff up until Warmind, after all he hears about is "omg SK is gonna be amazing", huge turnoff. So they let them dip their toes into SK stuff which would more than likely get them to buy into it. What we are also paying for when getting Season of Undying, are the memories and experiences that come with the new activities like Vex Offensive and the Exotic Quest for Leviathan's Breath. In the future, you *might\* see those pinnacle activities were exclusive to Season of Undying only. They mentioned in a ViDoc and Director's Cut; they want Destiny to be "A Single Evolving World" so certain activities exclusive to Undying may not be accessible anymore during Season of Dawn bc "the world evolved so its no longer there" but you can still get the weapons and gear from that Season, from drops or Banshee; but the "I was there when it happened" experiences are gonna be for those who bought into Season of Undying. That's what we pay for. *Remember, assuming we bought Shadowkeep which will include Season of Undying*
From that picture you can also see Festival of the Lost, Iron Banner, etc are not gonna be for Season-specific or Season Pass owners only. I believe that stuff will remain free for Base Players. Its been free since Destiny 1, the backlash would be astounding!
To finally end this response lol, the term Season Pass is dumb. Again, it meant something different back in 2009. We're used to hearing Season Pass and think "Oh we're gonna get like 4 pieces of huge content throughout the year!" If you're playing Borderlands 3 then yes; Gearbox still uses that content release model. It works for their type of game. If you bought their Super Deluxe Edition, it came with the game, some in-game items and a...Season Pass (which entitled you to 4 campaign DLCs).
For Bungie, as you may already know, its gonna be called the Shadowkeep Annual Pass which they havent said anything about (and rightfully so, they're probably still working on the last 2 Seasons for that AP).
The Season Pass, again, is like the Battle Pass of Fortnite, that doesnt give you anything other than a tier progression based something-to-do. Honestly shouldve been called like the Guardian Pass or something. I truly dont see Bungie making Destiny's "Battle Pass" offerings, anything other than in-game stuff like Ornaments, Engrams, Emotes and Mods.
Hope that cleared up any confusion; hopefully didnt cause more lol