Safari on iOS doesn't support extensions and requiring another app be installed can be frictionful. It is still early.

for nostr I agree is still early, but nos2x and Alby are doing great on desktop browser. I'll be not surprised if they extend to mobile soon.

maybe browser extension on mobile? Seen Nostore doing some interesting experiment on appl devices 

supratic

... and isn't encrypted local storage safe enough? 

Storing your nostr key for SN to sign stuff with. Storing E2E messaging keys. Basically any key material or sensitive data that you don't want stored on a server.