Ah, thank you for pointing these out! I'd missed the introduction of “flex” instance types (apparently in May last year[0] – still long overdue relative to the introduction of T4g in September 2020[1]). Curious that so far, they all appear to be Intel-based (C7i, M7i, C8i, M8i, and R8i). M7i-flex instances also cost 45% more than the corresponding T4g instances. That's sort of understandable, as the generational improvements probably bring more than 45% better performance for most workloads, but it also makes them harder to justify for the sorts of long-running,-mostly-idle duties they're being touted for.
If you're interested in the underlying technology of flex there's some reinvent talks from last year on YouTube where they acknowledge it's based on VM live migration which is I think the first public reference to AWS using migration in their products.
I suspect the burstable types were always priced too cheaply and were more about attracting the cheap market segment which they don't need now in the days of AI money.
Burstable pricing gets complex quick when adding in the option to burst to full usage. Flex seems a lot simpler which is great.
eg c7i-flex.large, etc.