We may never know why this feature is not included in certain models. I am reminded of something a top executive at AT&T once told me when I worked there. To paraphrase, he said whenever he was negotiating with someone (another company, a public official, etc.) he tried to never make any assumptions about motives, because when you do that, you are almost always wrong and it only makes it harder to come to a resolution. Instead, he tried to take everything at face value and negotiate from there.
Great advice. I see so many cases where people get caught up in assigning motives to Canon's actions, when we will probably never know why they made a particularly decision and, in fact, it doesn't matter.
The feature is not on the 90D, M6 or RP. Make your buying decision based on what you need and move on. Maybe at some point in the future Canon will explain their rationale. Or maybe they won't. In either case, it's not going to make any difference.