Yes, sorry I think I was a bit quick.
Sorting is not recommended for primary index because it usually “degrades” relevancy, surfacing most recent content over most relevant. You’re still free to do it, that’s why I said you can do it but it will really put a big emphasis on recent content.
Using custom ranking, we’ll only put the most recent content first, when 2 (or more) items are considered AS RELEVANT AS each others.
Generally speaking, I’d recommending using custom ranking over sorting for blog content but there are valid reason to use sort by. For example if the content becomes completely irrelevant after X days (event announcement for instance).
If you have an empty query (no search, no filters) then, the order will be the custom ranking, since they will all be considered equals. In this case, the order will be anti-chrono.
You get a red message when there is no custom ranking, because custom ranking really helps finding the most relevant and should always be set. It could be the number of comments or even a simple boolean to surface the “highlighted” posts.
Please let me know if that helps