How to understanding min/max input values of Schmitt Trigger from datasheet
The parameters you're looking for are VT+, "Positive-going input threshold voltage", and VT-, "Negative-going input threshold voltage". They are given in section 7.5, "Electrical Characteristics".

They have minimum and maximum values, and the appropriate boundary must be used for the given condition. The lowest voltage that would result in a guaranteed switch from low to high would be the maximum value of VT+ at 4.5V, which would be 1.9V. Likewise, the guaranteed high to low transition voltage would be 0.5V. It is possible to characterize individual parts to get more accurate values, but those are the values you can depend upon across the board.