Quote from Josh Noh, a developer,
"The turret slow does stack with itself, so 2 turrets will slow an enemy more than 1 turret will, unless that enemy is already slowing themself by a higher value. The new stacking rules are for splitting out friendly (ie. self slows) and enemy slows and taking which is greater.
So if Reinhardt has his shield out and is slowing himself by 50%, then you won’t notice him moving even slower until an enemy slows him by a value greater than that 50% (such as 3 Sym turrets hitting him).
The new rules were to help prevent heroes that slow themselves from effectively being rooted when affected by an enemy snare as well."