The refresh token by itself does not provide any information about the user, (the access token yes), so I don't really see the problem in storing it in local storage. However, I understand your concerns.
The issue is about letting the JS have access to any kind of credentials. Traditional session cookies are usually http only for that matter. The same applies to access tokens or refresh tokens.
It's just about avoiding attack vectors.
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IMO you should NOT store those credentials in local storage. Refresh tokens are supposed to only be seen by servers, aren't they?
Take a look at Laravel Passport approach: uses traditional cookies to store an access token and a CSRF token.
The refresh token by itself does not provide any information about the user, (the access token yes), so I don't really see the problem in storing it in local storage. However, I understand your concerns.
The issue is about letting the JS have access to any kind of credentials. Traditional session cookies are usually http only for that matter. The same applies to access tokens or refresh tokens.
It's just about avoiding attack vectors.