Is the slippery slope argument a fallacy when companies have shown that pushing their agenda piece by piece is their M.O.?

Reddit has already shown that they had the capability and the will to edit users posts, not just removing them. Reddit and other social sites already uses all sorts of algorithms to move content they want to the top and to bury content they don't want. Companies don't want negative PR about themselves or their products. Companies advertising on Reddit are Reddit's actual customers.

You put those facts together and being afraid of Reddit eventually censoring posts critical of customers doesn't seem like fallacious thinking.

You don't throw a chicken into a cage with a starving tiger and call it a slippery slope to say that the chicken is going to get eaten