I think you might want to rename this to --no-saml-soap-logout.

Why the ".rd"?

I'm ok to rename the option.

The .rd is basically a typo, probably a remnant of a previous comment I failed to clean up.

Maybe rephrase this to something like "Done logging out all sessions"?

I think this is a bad idea.
Instead, I would suggest you to make logout_mech=None and then have an "if not logout_mech: logout_mech = [].

There is a reason "[]" is called a dangerous default value :-)

Maybe rename to supported_logout_mechs or something else plural?

I think this might be a slightly confusing variable name.
Just having the join in tdata initialization shouldn't be too much a problem I'd say?

Wouldn't it be better to instead make this a prioritized list or something?
That way, the while loop that handles the actual logging out can be easier because it doesn't need the (in my opinion) confusing "if session is None: session = ..." at the end of the loop

This argument would DISABLE logout if provided, it does not accept any arguments (as it's a store_false).

+1

New patchset uploaded

I don't think this can pass pep8/pylint, does it?
I think it's kinda unclear where indentation starts and ends.

Maybe make this plural?

I've made a small change to the indentation, but otherwise looks good to me.