Tl;dr
My answer to whether we should continue having mobile article sections closed by default is a qualified no with 2 caveats:

1. We need to learn more about open/closed reading behaviours before making a final decision
2. Just opening all the sections by default won't give us an ideal reading experience either.

Here's how I got there...

I came across some old research in my desk research for next fiscal year's "content discovery" OKR. It found that "On 60% of mobile pageviews, not a single section is opened, i.e. the reader only looks at the introduction." We also know from this study and others that readers use infoboxes and TOCs as important navigation affordances on desktop. The team ran an A/B test of open vs. closed sections and found that open sections readers tend to read more, tend to engage with more content further down the page, and spend less time navigating. The team did 2 small qualitative studies (n = 7, n = 12) and found that their participants preferred the closed section reading experience overall, but also wanted some way to "zoom in and out" of an article via ToC, sticky headers, or some combination of the two (prototypes). At the time, design recommended sticky headers in Minerva, but those were never implemented, and the small scale qualitative studies outweighed the quantitative analysis.

Also, a recent bug found that the implementation of the section collapsing "hides" that content from some programatic readers like the Safari voice to text feature (see: T356091).

Just on principle, hiding article content is probably not a good idea.

Assumptions:

• Needing to tap a section to open it introduces higher cognitive load to read the content than just scrolling to it.
• Some readers want to view content that is currently hidden by collapsed article sections and do not open them because doing so requires more work.
• If sections were expanded by default, some of those readers will click on links in the content that is easier to access.
• The relevance and quality of a section title as a navigation signpost summarizing the content within that section varies widely.
• Giving easier access to more content in articles will improve "content performance"
• An effective mobile ToC will improve content performance.

Proposal:

• Review our clickstream data and see whether this trend has changed.
  • How often do readers open sections on Minerva?
  • Which sections tend to get opened?
  • How many sections tend to get opened?
  • How does article length impact these behaviours?
  • How does font-size preference influence these behaviours?
  • How does language influence these behaviours?
• Based on this analysis, formulate a hypothesis about how much more readers may read and how many more internal referrals may come from articles where we have sections expanded by default.
• We could also look at readers who have sections expanded by default in their preferences and see how different their reading behaviour is from people with the default, although that comparison probably isn't apples to apples.
• Run an A/B test to see if expanding article sections by default changes reading behaviours
• Design, test, and implement a table of contents for Minerva.

Open questions

• Why doesn't Minerva have a ToC? Did it ever have one? If so, what was it like and why was it removed?
• We might want to collapse some sections like references, citations, and external links.
• Does the auto-collapse feature impact pages other than articles?

Created follow-up tickets:

• Initial data analysis T366020
• A/B test T366024
• Design in-article navigation affordances for Minerva (ToC, sticky header) T366026

Moving to sign off for @ovasileva and @Jdlrobson