If users have to buy new hardware to get typing, or the other "old thing", as smoothly as it did, I would call it BS.
Forgetting the fact that it's a BETA, let me ask you a question. If you continue to use the phone you have for 10 years, do you expect that the 2024 iOS should still make your 10+ year old phone work just as good as it did originally? I hope your answer is no (it probably won't work at all) and I hope you would say no because it would be ridiculous to think a 10 year old phone could handle the freaky new software we'll one day have.
Software is getting more and more powerful and I DON'T expect it to be held back for any reason because it's trying to keep old phones working at 100%. So, yes, I expect it won't be as smooth. If it was my phone and it bothered me that much I will a) not upgrade or b) buy a new phone.
Simple solutions to a not unexpected technology issue.