I cant find anything online, does anyone know what it is?
Expect the answer to be close to sea-level pressure, as the F-1 is optimized for first stage performance.
The answer can be found easily by using the commonly cited statistics: chamber pressure and nozzle area ratio (expansion ratio).
The isentropic flow equations show that for a given nozzle area ratio there is a fixed supersonic pressure ratio. This ratio is also a function of the ratio of specific heats for the gas. For air, this value is typically taken as 1.4, however for combustion exhaust, this value is typically lower. I will take a value of 1.2 for my analysis (finding a better guess is possible if the O/F ratio, chamber pressure, and propellant formulations are known (which they are, but I am not opening that can of worms for this question)).
Wikipedia says the F-1 has a chamber pressure of 70 bars and an area ratio of 16. This gives a static to total pressure ratio of 0.0068. One more number crunch gives the nozzle exit pressure as 0.48 bars.
This is lower than the sea-level pressure I guessed in the beginning, but typical for first stage engines. 0.48 bar is the pressure at about 25,000 ft altitude, so the first stage likely operates beyond that height.
A published value for the exit plane pressure can be found in Comprehensive Review of Liquid-Propellant Combustion Instabilities in F-l Engines
I had done a fun calculation (hence the comments below), but the answer came out way too low.
This is close to the 0.5 bar mentioned in A. McKelvy's answer https://space.stackexchange.com/a/61261/6944