For the pH of a facial cleanser to be unsafe as such, it would need to be very acidic, below 3 or very alkaline, above 10, and even then, it wouldn’t necessarily be unsafe - as in, cause harm.
There is an optimum pH for products used on the skin and this varies from product to product and does depend on the skin’s pH too. It also depends on other factors that may make skin more or less reactive to the pH.
Most people wouldn’t know the exact pH of their own skin, so can guess that it is between 5 and 6, based on skin that has been tested. A cleanser’s pH doesn’t need to match the skin’s pH for it to be effective or well-tolerated. Having said that, if pH is the only criteria under consideration, I would produce one with a pH of around 5.5 - 6. This is acidic, but at the alkaline end of the acid mantle’s range. As it is a cleanser, it needs to have some form of detergent action and detergents tend to be alkaline (washing detergent is around pH10, bleach 13), if it is too low, it won’t clean effectively.
So, optimal: about 5.5, with other considerations factored in. Unsafe: a liquid is considered highly corrosive below a pH of 2 or above a pH of 12. Some skin tissue can be damaged at under a pH of 4 or over pH of 9. Obviously time left on the skin, temperature of liquid etc will all make a difference.