No, it isn't a limitation on the MCU1. There are plenty of MCU1 cars out there with 2021.40 software, even older ones with Autopilot 1. On Teslafi, over 30% of AP1 cars are on 2021.40 software.
It is a limitation on the configuration of MCU1 and FSD updates from AP2.5 to AP3 hardware. So few people have this configuration that Tesla won't bother with testing their latest software on this combination.
This isn't a hardware limitation. It is a Tesla not caring limitation.
I was getting regular software updates right up until I went in for the AP3 hardware upgrade. Not one software update since.
AP3 hardware hasn't added a single feature from my old configuration, and has effectively frozen any software bug fixes and feature improvements. Had I known at the time that I would gain nothing and lose much, I would have never upgraded to AP3 hardware.