Numbered headings runs on Google Cloud. Google Cloud has published a security statement, which you can find here: Google Cloud Security Statement.
Numbered Headings Cloud processes your Confluence data to number your headings. This data is stored within Confluence.
Your Confluence data is transferred but not stored, cached or otherwise retained outside of Confluence.
We make sure that our products are as secure as possible by attending to the following things:
TLS 1.2+ encrypted connections to and from our service.
HSTS to prevent downgrade attacks and cookie hijacking.