Is it? As far as I understand it is a philosophy. Like many ideologies, it got turn into a religion afterwards
Taoism originally developed by Laozi(https://en.wikipedia.org/wiki/Laozi) as one of many ideologies in Warring States period(~4 BC)
It became into a religion starting from Han Dynasty(~1 AC)
[0] I think you still see this mix-up today due to a lot of people reading the TTC or the Zhuangzi from Gutenberg or some other free online translation, not realizing that all those translations are from the 1800s and not a reflection of modern scholarship. For example, Laozi hasn't been considered a historical figure for a while now.