I have my bitcoins in cold storage but I am using a read-only wallet with Electrum. Is that safe from hackers?
Depends. How did you generate the wallet? Did you properly segregate your cold wallet from any internet connection? Did you validate your Electrum instance?
My other question is:
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?
Your seed phrase is used to generate the private key, you will always be able to generate the private keys for the corresponding addresses. There is no way to disable this feature; the seeds generated by Electrum is only supported by Electrum but since the method of generating the BIP32 seed (and subsequently the addresses) is known, there is no risk of you not having access to your coins. Of course, barring any sudden network changes that prohibits the address standard you're using which probably won't happen.