I have a 22 year old chartermi.net email address. They eliminated those 16 or 17 years ago. Still works fine. Supposedly when they dropped adding the state intials to the end, those accounts would all be switched to charter.com.
When they tried to do this, they failed miserably, and so the accounbt remains. Even though all new accounts are spectrum.net.
In other words, I don't think you have anything to worry about, it should work fine.