List View Threshold changed from a maximum 5000 items per view to greater than 5000 items. Yes, you are reading it correctly, Now it is more than 5000 items in a view but what is an upper limit? Sorry, we don't know.
Upper Limit: It depends on the SQL hardware, SharePoint server's hardware, and structure of site collections.
Now they introduced new values for the following.
List View Threshold: Greater than 5000. Specifies the maximum number of list or library items that a database operation, such as a query, can process at the same time outside the daily time window set by the administrator during which queries are unrestricted.
When adding or removing a column index, the threshold is 20,000 by
When deleting a list or folder, the threshold is 100,000 by
When renaming a folder within the same library, the
threshold is 100,000 by default.
List view threshold for auditors and
administrators will remain same at 20,000.
List view lookup
threshold: 12 join operations per query
Reference is for SharePoint 2016 OnPremises, but this also applies to SharePoint Online.
Another thing is a different threshold between SiteCollection Admins and normal users:
List View Threshold - 5,000
Specifies the maximum number of list or library items that a database operation, such as a query, can process at one time. Operations that exceed this limit are blocked.
To give you time to make alternative plans, you may be warned on the List Settings page when your list has exceeded 3,000 items. The warning contains a help link to this topic.
List View Threshold size for auditors and administrators - 20,000
Specifies the maximum number of list or library items that a database operation, such as a query, can process at one time when performed by an auditor or administrator with appropriate permissions. This setting works in conjunction with Allow Object Model Override.
Threshold is exceeded (>5000 elements) but View still working. Why?
Manage large lists and libraries in SharePoint