I have a mid-2016 MS90D, so AP1 and as of today still on MCU1. What I am seeing now is that software updates for AP1 MCU legacy systems appear to be coming roughly once a quarter. For those that want the specifics, dates I've installed last 4 update are: Feb 18, 2021; May 12, 2021; Jul 24, 2021; Oct 15, 2021.
For the most part these have tended to be mostly bug fixes, but have been a few "feature" changes per the release notes, such as improvements around schedule departure functionality, supercharger info display, vehicle information display info, ability to stay connected to wifi. Some would say those are nothing significant, and clearly not some of the features being delivered for MCU2 cars such as the most recent abilities to better control charging via the app, ability to add waypoint to navigation, etc. While I'd like to get some of those, I also understand the realities of all companies needing to make decisions around product obsolescence timing and when they start to scale back/limit updates for older generation products.
Personally, I've come to accept that this is the situation for MCU1 with roughly a quarterly software update cycle being reasonable and understandable in my opinion. Whether this will continue to be the cadence over the next year, none of us really know, but I know about 2 weeks from now it won't matter to me personally as I've finally decided to pull out my wallet and have an appointment scheduled for the MCU2 upgrade for my car.