The undertale_(series) tag isn't quite the same as the undertale tag. The former covers both undertale and deltarune while the latter just covers undertale. But it's not used as much as it needs to be, and going through over 10000 posts manually to add it is uh... Impractical to say the least. Is there any way it can be done automatically?

General users being able to do that would result in it being fat too easy to troll the site with false tags. It can and will need to be done manually, if at all. Browsers could achieve the same effect simply by searching be the of those tags, too, so it's a lot of effort for pretty much nothing.