Deleted scenes for Frozen 2 and the upcoming book both suggest there was a strong anti-Northuldra sentiment in Arendelle, to the point where it's implied it could have costed Iduna her life if they found out her true identity.

Is this a good justification for Iduna's actions?

Should they have kept this aspect of Arendellian society in the film?

Did it serve any purpose at all for the plot?

Frozen 2's deleted scene: Secret room:
https://youtu.be/_AxYnIdROiQ
>Norhuldran language is forbidden.

Frozen 2 Dangerous Secrets' summary:
http://archive.is/yZqVI
>Ever since the day the forest fell, Arendellians have despised and distrusted Northuldra with a vengeance.
>Fortunately for her, Agnarr doesn’t know that Iduna is the Northuldra girl he saw seemingly flying on a gust of wind all those years ago, the day of the celebration turned disaster. The day Agnarr lost his father, the king. The day Agnarr himself almost died.