The Advanced Settings for Sites article has the most up-to-date information on this topic.
DubBot has added the option to open and crawl page content that is hidden by accordions. It can be set up in the Site Settings area.
Locate Page Preview Options
Go to Settings (located at the top right-hand side of your screen).
Choose a site from the sites that appear as the first item in the Settings menu. In that site, click on Advanced.
Scroll down to Page Preview/Analysis Options. The first field Selector to click on page load, is where you can add your accordion selector. If you have a selector in that field already (to close banners or popups), simply add a comma behind the current selector(s) and add another one for accordions.
Example Selector
Here is an example of an accordion selector that could be placed in this field:
.accordion-item button[aria-expanded='false']