That was for Admin Tools 2.0 released in 2010. That feature was removed around 2012.
Simply put, combining and minifying files is something that needs to be tested on a page by page basis. People are not willing to do this. Also, that feature may have made sense in Joomla! 1.5 through 1.7 but with 2.5 and especially 3.x it made less sense. Any template worth its salt uses LESS, SCSS or something similar to produce a single CSS file instead of using 15-30 different files. Also, since Joomla! 3.1 you can override JS and CSS files and even prevent files from loading, making it possible to do combination and minification in the One, True Way: by hand. Moreover, this feature didn't really make sense in a security product. Therefore this feature was removed and it won't come back.
Nicholas K. Dionysopoulos
Lead Developer and Director
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