I'm sorry, but an assist CAN indeed by credited on a play in which no putout results. And yes, this applies to MLB, college, and high school.
MLB 10.11
An assist shall be credited to each fielder who throws or deflects a batted or thrown ball in such a way that a putout results, or would have resulted except for a subsequent error by any fielder.
NCAA 10.14. a. An assist is credited to a fielder when he handles or effectively deflects the ball during action that is connected with a putout or he handles the ball before an error that prevents what would have been a putout.
NFHS 9-5-3
An assist is credited to a fielder each time he handles or deflects the ball during action which is connected with the putout or he handles the ball prior to an error which prevents what would have been a putout...
Along the same lines is the old and oft-repeated canard that you "cannot assume a double play". Well, yes and no. You cannot assume a double play if, for instance, the shortstop fields a ground ball, tosses it to the second baseman covering the bag, and the second baseman throws wildly to first. But you CAN "assume" a double play if the second baseman's throw is accurate enough to catch, but the first baseman drops the ball. Now you have an assist for the SS, both a putout and an assist for the 2B, and an error for the 1B. If the double play would have ended the inning, any runs scored afterwards and charged to the pitcher currently in the game would be unearned.