If your premium subscription expires, your TOTP authentication keys will remain in your vault, but the Bitwarden clients will no longer generate the TOTP codes. Once you renew, the ability to generate codes is restored.
You can store the TOTP seed in Bitwarden with any client. Premium activates the generation of a TOTP from the stored seed. If you do not extend Premium your seed will still be stored, you just won’t have any codes generated until you activate Premium again.
Your question prompted me to check out my current TOTP setup. I have six TOTP’s setup. Five of them have backup 2FAs (text to my cell phone or email). The one that had no backups at all I moved back to sms so a situation like this could not occur.