Liquid chlorine has a high pH of about 13. It temporarily raises the pH of a pool, but the chlorination process brings the pH back down and neutralizes it.
The short answer is while liquid chlorine temporarily increases the pH of water, the net pH impact over time is almost neutral.
This occurs thanks to the sodium hydroxide in liquid chlorine:
NaOCl + H2O → HOCl + Na+ + OH-
Sodium hypochlorite + water → Hypochlorous acid + Sodium ion + Hydroxide
Sometimes, the above equation is expressed with the result of HOCl + NaOH, which is sodium hydroxide. The hydroxide is a base, and that temporarily raises the pH of the water.
BUT, then chlorine begins to do its thing....
Chlorine produces its own acid
In the equation above, the HOCl product is the killing form of chlorine (hypochlorous acid). It is a weak acid, but when it kills or oxidizes contaminants (or gets destroyed by solar UV radiation), it loses its Oxygen and forms a strong acid, Hydrochloric acid (HCl). Hydrochloric acid is better known by another name: muriatic acid.
Credit: Robert W. Lowry, used with permission
Indeed, chlorine products produce their own Hydrochloric acid as an essential byproduct of chlorination in water.
Is acid needed to offset the high pH of liquid chlorine?
Standard industry practice has been to add some muriatic acid each time liquid chlorine is used, in order to offset the high pH of sodium hypochlorite. This practice can potentially be beneficial if the acid is properly measured and diluted prior to pouring, and if it is poured prior to adding the chlorine. But no, it's not always needed. It depends on the pH of the water to begin with.
If the pH is over 7.8 when you are adding liquid chlorine (or cal hypo, for that matter), the temporary pH rise may cause some cloudiness to occur. Consider lowering the pH below 7.8 prior to adding hypochlorite chlorines.
NOTE: Never mix acid and chlorine together; it will create dangerous fumes.
Instead of thinking of acid as a requirement to offset the high pH of liquid chlorine, it's helpful to focus on the total alkalinity and pH ceiling in the water prior to adding the chlorine. Acid should be used to augment those numbers so the pool can be maintained in favorable LSI balance for the next 7 days (assuming the pool is being treated once a week).