Dragon Age: Veilguard does not warn you about which side quests will prevent you from completing them.

I was not that far into Dragon Age The Veilguard before I realized I had been locked out of an unexpected sidequest. After the first significant event, you have to make an important decision that will have long-lasting effects. If you don't pay attention to your quest log you might not notice some quests disappearing into the completed section. They have a little note above them that says something like "You cannot complete this quest because this event has occurred".

This happens throughout the game and is most noticeable in companion and faction quests. These are the two main quest types to complete if you're looking to gain allies, faction rep, or romance a certain character. Taash is one to watch out for.

You won't have much time to complete her companion quests. These include Fire On the Sands, and A Little Dragon Hatches before you can move on with the main story. You may have a harder time romancing Davrin if you don't complete her companion quests. Davrin’s first companion quest may also disappear if it is not completed.

Some quests may disappear based on your choices. For example, the faction quest A Slow Poison can be terminated by the first major decision you make. This same choice opens up other quests, so you don't have to be a completionist.

I won't spoil the details of major in-game events but I will give you a list of quests and quests that can be locked by each event so you know whether to save them or finish them.

• A Warden’s Best Friend :Complete a Slow Poison plus any subsequent quests before this.
• A change on the wind: Complete Davrin’s first quest and Taash’s Fire On the Sands prior to this.
• A Fighting chance:Complete Taash’s A Little Dragon Hatches Before this date
• Blood of Arlathan - Complete Davrin’s Hope on the Wing before this event.

These are the main companion quests and faction quests I found. There are likely more, since they are often unlocked in order. If you pay attention to the quests that tell you to gather all the characters in the Lighthouse they are usually important and move the plot forward.