damus.io/key can be used offline for greater security, the page works offline.
You can also use https://damus.io/key/ if you’re just talking about your public key. You’re not leaking private keys.
Discussion
If I only have to paste the npub key there online would be just as safe. As long as private key is not online ;)
You don't need to be online, you access the page, then you disconnect from the internet and then you can add the public key to get the conversion offline. Even being offline you can refresh the page and it works.