I have imported an excel file with 12000 items into the Sharepoint List. The default view is All Items and I have not got any 5000 Threshold Errors, though. Probably this is because I didn't scroll (by 30 items in a batch) to the bottom.
The Automatic Indexing creates the first index and all subsequent indexes are created after sorting any columns (besides those that are not supported). So it is not necessary to create them manually.
But List view threshold: 11282 items (list view threshold is 5000) is still there (in the list settings).
After some googling, I have found this article it helped me to realize how ridiculous this 5000 throttle is. If I understand it correctly, to work with lists longer than 5000 items, I need to create separate views based on some filtering, but how am I supposed to know that that filter will return less than 5000 items? Microsoft proudly inform their users of 30 millions list items limit ...
Having Excel files on Sharepoint online to mitigate this issue is not an option for me.