The wifiwave2 L2MTU was limited to 1560 bytes in RouterOS 7.12 (2023-Nov-09), pending a bugfix.
https://mikrotik.com/download/changelogs?channelFilter=stable
Is this bug still around? If not, can this restriction be removed?
I'm not sure what's the driver limit for MTU. However, 802.11 standard says 2304 bytes is largest L1 payload "over the air" and there are some overheads to be accounted for (such as WPA2/WPA3 encryption overheads, typically around 20 bytes). So maximum L3 MTU is not that much higher than standard ethernet MTU of 1500 bytes to be worth bothering IMO.
And most often having really large packets transmitted over RF can prove counter-intuitive ... due to relatively high probability (compared to wired technologies) of packet become corrupt e.g. due to infrequent short interference burst, the whole large packet has to be retransmitted (and that takes quite some air-time) while with standard-sized packets lower proportion of somehow smaller packets have to be retransmitted.
It was limited to 1500 MTU when it was still wifiwave2. I have not tested this is a while but I want to say it allows more than 1560, but the specific hardware might matter IDK since wifi-qcom-ac is actually multiple drivers.
But to @mkx point, you really only want to run a higher MTU on the Wi-Fi interface is running a tunnel over the link (e.g. PtP/PtMP/"mesh" using something like VXLAN, EoIP, IPSec/WG/"VPNs"). And 1560 should be enough for any of those. I'm not sure the use case beyond the "baby jumbo" (e.g. enough to carry outer headers of tunnels).