Even running on AWS is not completely isolating you from hardware related changes. For example AWS is retiring both old instance classes for databases and Aurora 1 soon. This is a multi week migration project with a hard deadline for me and it's not even a huge company/service. For Twitter I suspect that kind of deprecation would take month(s) to fully roll out.
That's surprising to me, given how much effort AWS put into emulating old instances types using newer instances to still be able to offer instances with the characteristics of old instance types to customers. [1]
Do you have additional information about that you can share (maybe an announcement regarding this made by AWS)?
It happens quite often. I'm dealing with deprecation of db.r3 for example which will be enforced in a few months. (I easily find the public announcement - you get an alert if using it) The instance sizes don't cause that much problem as such, but you still need to spend time assessing the new performance, updating reservations, figuring out the transferability of the old ones, etc.
