This actually isn't true for modern variants of Tetris. A correct implementation of the random number generator requires that the 7 pieces be shuffled into a bag and then drawn randomly, ensuring an even distribution, and disallowing long runs without a desired piece. (Maximum of 12.) The bag is re-shuffled every 7 blocks in the sequence.
https://tetris.wiki/Tetris_Guideline
https://tetris.wiki/Random_Generator
Thus, in a game of Tetris adhering to these official guidelines, the longest sequential run of S and Z pieces would be 4, in the following sequence:
[5 Random Blocks] [Z and S in any order] [Z and S in any order] [5 Random Blocks]
I'd be very curious to see a proof of an impossible sequence which adheres to the official rules, as that would be quite a feat indeed!