This has been removed as well as some other buttons, but we have a list of such "missing" things here and there, and we are treating these as bugs. We will add them back.
We haven't been able to work on this yet. We apologize for the inconvenience.
The only way I found is to switch to Grid view - up to the right of the page there are 2 buttons - and then you can click the column names for Username, Name, etc.
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