For what it is worth, AT&T just deprioritize you if the cell tower you are connected to is congested. Otherwise, your data will not be capped or throttled.
If you have a smartphone and exceed 22GB of data in a particular billing cycle, your data speed may be reduced during the remainder of that billing cycle if and when you use your phone at times and in areas that are experiencing network congestion. You will not experience any reduction in speed while using Wi-Fi.
The hypothetical plan you mentioned would be a hit in Kenya at that price point (-/+ 500)
Quick questions: - How much do you rely on your phone data bundle?