I personally have no experience with MediaTek devices. I recently considered buying one, but skimming through some of the support threads here about it, it's wifi support or performance just didn't seen as mature and reliable to me, so I dismissed the idea. Your mileage will vary, obviously.
Nevertheless, I looked at the kernel log and it seems at least some supported MediaTek devices should support both MU-MIMO and beamforming, namely those using the mt7915 wireless chip, see:
(Note: I only looked at the kernel tree up to version 5.10 since this is the version OpenWrt currently backports its wireless drivers from. It might be well the case that newer kernel releases and OpenWrt versions will have MU-MIMO/beamforming support for other MediaTek devices as well.)
The kernel log also mentions support for mt7615 chipsets which support MU-MIMO/beamforming as well. But I couldn't find any hint as to whether those features are actually supported by the driver. But since OpenWrt developer @nbd is listed as one of the maintainers of the MediaTek kernel support, maybe he can chime in and give a more accurate assessment of the MediaTek Wave 2 wireless support in general.