so it's not as bad as other games, in fact, it is one of the best in terms of lack of fomo
I can list in one post all the forever missable stuff
1. legacy titles and the legacy tattoo: small coametic things you could only get during an event in 2012 assuming you played in the 1.0 version
2. the regalia mount and noctis lucis caelum outfit: will probably come back in a few years
3. pvp the feast outfits and mounts: will eventually become purchasable using pvp currency
4. pvp series 1, 2 and 3 rewards: also will eventually be purchasable with pvp currency
5. promotional mounts and emotes, merchandise only items such as matoya's hat: eventually purchasable via the square enix merch store but will cost you a pretty penny

I think that's it
every single other mount, minion, cosmetic outfit and emote is acquirable ingame or via the cash shop