@Psychronia I mean, it makes sense that tsunderes have probably always been a thing in literature. It's likely that they've just gone by other names or didn't have a proper title attached to their attitude.
Edit: Also, as some others have mentioned tsunderes have shown up in manga since at least the 70's, if not earlier.
@Pyoro I'm still looking for the shojou-ai tag on the description page, but no matter how many times I refresh the page it doesn't show up. Clearly Mangadex is borked.