Discussed ExpandingSection topic in the community meeting today, but summarizing some design aspects here also.
ExpandingSection is not documented, but allowed component that is part of public API. So far ExpandingSection has seen limited usage, inside platform we have only used it to implement few dual SIM settings. Often when user opens a particular page you want to show all the information in full instead, and allow scrolling through all the content without requiring taps in between to open sections. People app used to have alphabetical grid, which hid all the contacts even when user didn’t have many contacts or alphabetical groups. In the new design you can both scroll your contact list or navigate straight to the section using the scrollbar
Expanding sections has its uses though, if you have repetitive content under many sections. Though if the sections grow big you may want to consider opening new pages instead.
Further expanding sections can be confusing to the users with many animations going on simultaneously (especially if you have large sections there is a lot movement): one section is expanding, other one is collapsing, one section header is moving to the left, the other is moving to the right, all the while the page position is getting updated. Frankly the animation would need a bit of work so it works properly with different number of sections and different section sizes.