Exclude certain entry types from Entry URL format? I’m trying to find a way to ‘group’ pages in a structure, where that group page has no function other than to visually organise the entries in the structure, and does not need to be part of the url structure.. eg:

- Major Campaigns 
   type: landingPageGroup
   slug: major-campaigns

    - Moving Office
       type: landingPage
       slug: moving-office
       uri: /moving-office

        - Your Moving Checklist 
           type: landingPage
           slug: your-moving-checklist
           uri: /moving-office/your-moving-checklist

- Major Events
   type: landingPageGroup
   slug: major-events

    - Conference 2016
       type: landingPage
       slug: conference-2016 
       uri: /conference-2016

        - Agenda 
           type: landingPage
           slug: agenda
           uri: /conference-2016/agenda


If I set the Entry URL Format to:

Top level: {% if entry.type != 'landingPageGroup' %}{slug}{% endif %}

Nested: {% if object.parent.type != 'landingPageGroup' %}{parent.uri}{% endif %}/{slug}

...the URLs look correct in the entries listing, but it will throw a 404 when you try to access them... is there anyway I can get around this with some custom routing maybe?
I don't know of good, non-hacky way to do what you're doing.  Every entry in a structure should have some meaning otherwise you're going to fight the system.

Is there a reason why those top level landingPageGroups are just "vestigial" and serve no purpose? That would make them good candidates to break into separate sections. So Major Campaigns would be a new structure section, Major Events would be another new structure section, etc. 

You could probably hack your web server to rewrite slugs so that /major-campaigns/moving-office becomes /moving-office but for every section you add, that becomes a nightmare.

Even if you think you'll use one template across the board, it's inevitable that at some point you'll be using different templates for each "section" as well. I don't know your content strategy but if you only use one section, your entry types will likely multiply too.