This is the FOMO that exists in the game.
  1. Seasonal events. The quests and rewards are no longer available after the event is over, although often the rewards get put on the mog station a year later, so you can pretend you didn't miss the event if you are willing to pay. The rewards sometimes include mounts though.
  2. PvP has always had FOMO rewards in the form of top 100 rewards, but this recently changed to be series rewards. This is still FOMO, but you have a whole 4 months to get them.
  3. Housing, because if you own a house (which you don't need to), then you have to keep logging in or it's destroyed so that other people can build one.
  4. To a lesser extent, raiding is FOMO because hardly anyone will do it in its challenging form after the raid tier is over (after 8 months). It is technically possible to do it with the minimum item level setting if you can get 7 people to help you though.
The last two are a stretch. The seasonal event stuff barely matters. PvP is the worst one. So it's not really that bad in this game.

Seasonal events take literally 30 minutes of your time, out of months between events. It's the most casual content in the game that even people who don't have time for any other content do. You don't need to bother with a house. You can build a static of 7 friends to do old raids with if you put them off for 8 months and miss out.

So it's really just PvP and you can ignore PvP if you want. You may miss out on mounts, minions and glamours from it but you already do, because some rewards like this are exclusive contest rewards for gear/weapon design contests, buying limited-edition music albums, or participating in region-locked events involving retailers such as Amazon or food companies, where the retailer gives you a code to unlock a mount after buying something, or for watching a twitch stream. These events are rare but it means you have to accept there are some rewards you just can't get.

If you are trying to collect all the items in the game, collect websites filter out these time-limited rewards anyway so that you don't have to focus on them.