I'm curious, is there a setting or GUI option visible that says so?
If so, can you show a screenshot, please?
(Reason: this is a quite common technology, I wasn't aware it was still used as a marketing term - I want to make sure I didn't miss it on my devices.)
Well, I know it needs to be using at least 2 antennas, and the client must be at least 802.11ac.
This would be displayed by the RX or TX as a VHT-NSS value >= 2 on the OpenWrt display. AC clients will only show a TX value of 2, as they only RX beamformed signals.
You at least need an ac client (I don't have an OpenWrt with an ac-only chip to verify from the AP's perspective - I tested an ax, setting it to ac). AC was able to send to clients on their downlink. I set an OpenWrt with ax radio to ac to test - it continued to give VHT-NSS 2 on TX to ac-only clients (their downlink or RX).
"With the development of Wi-Fi 5 in 2016, there is now a set of specified beamforming techniques for Wi-Fi equipment that allow it to interoperate in a vendor-neutral way"