Right now we have Season Exclusive:
* Iron Banner Emblems
* Clan Banners
While it hasn't been explicitly stated yet, the general consensus is that once Season 3 rolls around, none of the ornaments introduced in Season 2 will be available anymore.
While I can understand why something like this might exist (to give players a memento for being here during a specific part of Destiny 2's history), I disagree with it on a fundamental level.
In case nobody knows what I'm talking about, I'd like to remind all of you of the various atrocities that plagued the game and also these forums during Season 2 (Curse of Osiris, Dawning, Faction Rally, etc.). A lot of people were here to experience those things as they happened, and a lot of those people decided "I'm leaving D2 and not coming back until it's better". The thing is if they leave and don't come back until Season 3 or beyond, there's a lot of this stuff they're going to miss out on. So many ornaments they'll never be able to get, an Iron Banner Emblem that's rendered permanently unavailable, and a color of clan banner that...matters much less than the other two things on this list.
I believe that while the drop in players is bad, the decision to leave is perfectly understandable given all the stuff that's been happening. Players shouldn't be punished for leaving, they should be welcomed back with open arms, no matter how long they've been away.
Keep limited-timed exclusives to events like the Dawning, SRL, Crimson Days, Festival of the Lost, and Moments of Triumph. They shouldn't be happening with Seasons.
To keep things interesting, I believe that the requirements for all ornaments should be reset when a new Season comes around. E.g. You are on 23/25 heroic strikes when Season 3 arrives, you are brought back down to 0/25 and must start all over again.
You know, destiny isn't the only franchise that does this. Other companies do it too. It's not a thing that's unique to this game. For example, watchdogs 2 has season exclusive rewards you can't get once a season has ended. It's one of those things added to get people to stay and actually play the game. I'd wager that the player base would drop so much more if they removed season exclusives as they wouldn't have much incentive to play - can't see it changing anytime soon.