If the purpose of this is only for the product manager to see how changing the value affects the display, then I suggest that we don’t submit this as a config change, but only temporarily edit the config file on a debugging server. @Lydia_Pintscher can we find a time window where we can try this today or tomorrow?

Alright, we figured out the issues and managed to test this prospective config change on the debug server. In short: yes, it seems to do what it’s supposed to do, let’s enable that setting on Wikidata (i. e., remove the line which sets it to false so that it uses the default value of true). It’s a bit tricky to test – MobileFrontend seems to cache the value of the “Expand all sections” user option in the session, so after practically any change you want to open a new private window to ensure you’re really getting the server default – but setting wgMFCollapseSectionsByDefault to true changes the behavior of talk page sections as desired (they’re collapsed by default, unless you change the user option), and nothing else we found seems to be negatively affected (in particular, mobile item and property pages look fine).