Is it possible to install WiFi package on L009UiGS-RM (ROS 7.13+) and use it as capsman controller for several hap ax for wifi6?

Yes

Any RoS device should be able to function as a capsman controller was my understanding.

Requirements
Any RouterOS device can be a controlled wireless access point (CAP) as long as it has at least a Level 4 RouterOS license
CAPsMAN server can be installed on any RouterOS device, even if the device itself does not have a wireless interface

Is it possible to install WiFi package on L009UiGS-RM (ROS 7.13+) and use it as capsman controller for several hap ax for wifi6?

Capsman in 7.13+ is part of core wifi functionality which is installed always. What is then left to install (as ootiobal package) are appropriate drivers for wireless chips. L009 doesn't gave eireless hardware, hence installing those packages would be useless.

So no need to install anything to configure L009 as capsman for ac/ax cAPs.

Thank you

Yep, you can totally add a WiFi package to the L009UiGS-RM. Just double-check compatibility.

Yep, you can totally add a WiFi package to the L009UiGS-RM. Just double-check compatibility.

But you should NOT unless for legacy capsman.
There is no radio on that device and as indicated above, support for wave2 capsman is default present as of 7.13.

There is actually a valid usecase for installing wifi drivers on the capsman controller without own wifi interfaces: You can use it to update your caps if they share the same architecture.

You are correct (almost forgot about that one) but even then I prefer not to do this.

I keep my install files on a separate USB so I know what's going to be installed

I haven't tried the new capsman (yet; I only have one wave2 device running wireless at the moment), but in old capsman one could configure location of upgrade packages ... which could contain files for different architectures. And CAP upgrade would then still happen automatically. And, as @holvoetn mentioned, that location can be outside the built-in flash (e.g. external USB key).

And CAP upgrade would then still happen automatically.

Depends on how you set that parameter.
You can choose not to let it be handled automatically (I don't use automatic upgrade. _I_ will decide when an upgrade happens on my devices, nobody else).