best display filter to find open wifi networks?

edit

Thank Bob for your answer "fixed" do not exist as an attribute in my version (2.2.17). Do I need a special plugin? There should be a simpler answer since simple programs are able to display an open padlock without requiring a plugin.

No plugin should be needed.

Not knowing why you need to stay on a dated version, I suggest you upgrade. If you can't/won't, then it useful to know that filter syntax comes and goes, as this link documents:

https://www.wireshark.org/docs/dfref/#section_w

I have found the same field in a 1.9.x version we have (for a specific use case) as

wlan_mgt.fixed.capabilities.privacy

Per the docs, this is valid for version:

1.0.0 to 2.2.17

Thanks again Bob
The "_mgt" was missing in your first answer. Now your solution works.
The description presented by Wireshark dissector is very misleading ("Privacy: AP/STA cannot support WEP"). It was an appropriate description in 2002 but the latest edition of 802-11 do not mention WEP anymore. The bit now means "encryption required" and is only used by AP (not STA).

But I feel there is a distinction between "unencrypted" and "open" A network may be unencrypted but require a login password.
So, my initial question is still unresolved.
But thanks anyway. I learned about the privacy attribute.