It's not necessary safe if the app is on the App Store. Apple don't check if the app is misusing the nsec. You are assuming trust of the app, the best way to check it is safe is if the app is open source so it can be verified.
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thx for replying i was using nostrmo from the appstore its not opensource (yet) according to the dev, its good app and liking it a lot but now im having doubts about using it here's the link https://apps.apple.com/id/app/nostrmo/id6447441761