I have a library with over 10k items. We are migrating from 16 to Online. I created a template from the library, did not include content. In Sharepoint Online i indexed the necessary columns for filtering, went so far as indexing 20 total columns, but it still isn’t working. It still shows I am exceeding the threshold. Even on views that I know should only have 5-10 items it still shows this. Any ideas why this is happening? Thank you.
List view threshold is a known issue in SharePoint online, try to follow the below :
Set filters through "Modify list view" and limit the number of items in the view to less than 5000 to mitigate this issue. You can categorize your list items based on metadata whichever present in the list such as category, department, business function, etc.
Archive old list items either by moving to a new list or a folder
inside the same list.
Try adding Indexed columns.
More details how to deal with threshold issue :