I have my bitcoins in cold storage but I am using a read-only wallet with Electrum. Is that safe from hackers?
Yes. Your watch-only is safe because it does not store seed phrase, neither did it store any private key, only your master public key was imported and stored which can not be used to access your Bitcoin. As the name denote, watch-only, it can not be used to sign a transaction.
I have an extended seed phrase, more than 12 words. Is it safe for future updates in the bitcoin network? Or is it possible that this feature will be disabled in the future so I can’t access my wallet anymore?
The 12 words are not extended words, it is the seed phrase. Store it very well as it will be required for wallet recovery. You should not bother about any changes in the future, even if there are certain implemention, it should not affect the seed phrase recovery process. But always keep watch of what is going on about Bitcoin best answered this.