Landsat collections with Landsat 8 data (landsat-ot-l1 and landsat-ot-l2) have been expanded with Landsat 9 data. The satellites have similar sensors, so that’s why their data is grouped together by USGS - data provider, and us also.
We are in the process of mentioning Landsat 9 in our docs and apps. Until then, do note that Landsat 8 docs and options apply to Landsat 9 as well.
