This is not Apple Support. This is a user to user only forum and we have no clue when Apple may drop support for iPhone 11. We can guess that it will still be a couple years or more, but we can't answer the question definitively.
Typically, an iPhone model becomes vintage when it hasn't been sold for more than 5 years and obsolete when they haven't been sold for more than 7 years. Vintage phones can be serviced as long as there are parts still available. Obsolete phones are no longer eligible for service.
That considered, iPhone 11 was discontinued by Apple in September 2022. It would be logical then that iPhone 11 won't enter Vintage status until 2027/28 and Obsolete status until 2029/30. But this is all a guess based on past history.
Please note since you are planning to have batteries replaced, the ONLY place you should do that is with Apple or an Apple Authorized Service Provider. Letting anyone else open the phone and replace parts will ensure you don't get authentic parts and will render the phones ineligible for service from Apple ever again.