Metal spoons aren't the only problem though. I read a study once that said specific stainless steel grades can leach nickel into acidic ferments over time, messing with the bacteria balance. So even a "clean" spoon might be doing something weird to your batch without you noticing.
My grandmother lost a whole crock of sauerkraut that way back in the day. A tiny scratch from a fork can let in the wrong microbes. I use a clean wooden chopstick now for tasting my kimchi. It feels like such a small thing, but it really does make a difference in keeping everything safe.
Come on, it's cabbage. People have been making this stuff in dirty cellars for centuries. I just use a clean spoon from the drawer to taste my ferments. Never had a batch go bad. I feel like we overthink this stuff now with all the gear and rules. My great aunt used her hands to mix it and it was always fine.