Quoting the old wifiwave2 docs:

802.11n/ac interfaces do not support this type of VLAN tagging under the wifiwave2 package, but they can be configured as VLAN access ports in bridge settings

(There's a similar warning in the new docs.)

Is MT going to implement the ability to configure vlan for ssid by capsman for old ac devices, or this setting permanently lost with the new driver?

I have asked support and answer I get is "It is currently not supported"
My guess is that not enough AC interfaces where running the wave2 package with capsman (too few complained) but 7.13 will change that so i hope it will be fixed.

Yeah, I'd like to know too. I've spent almost a day trying to figure out if I could manage old cAP ac devices with qcom-ac drivers in new WiFi CAPsMAN and couldn't find any reasonable solution. I understand that I can bridge WiFi interfaces to an appropriate bridge locally but how to do the same using new WiFi CAPsMAN? Before VLANs will be supported in qcom-ac drivers I don't see any benefits in using new wireless driver because one SSID just isn't enough.

VLAN do work but you need to configure them manually on the bridge on each CAP.
Check this thread out: viewtopic.php?t=202565