Many fermentation processes produce gas, and some aerobic fermentation processes happen while fully exposed to the air. Gravity provides quite a bit of assistance separating gas bubbles from solids and liquids.
In space, how do you build a fermentation airlock? With a centrifuge? Can you safely punch down dough? How do you open a sealed, pressurized container of anything carbonated? Can you easily remove enough dissolved CO2 from your kimchi so that eating it won’t make you burp?
why you need a airlock? just burb regularly by opening the can. you can also press the miso towards the bottom with some spring mechanism attached to the lid