man i can't even imagine how lazy the site admins are considering there's language filters already AND the previous site had this feature. you would think this could be done in a day or less.
You would think incorrectly. None of the site's previous version's code was reused, so the effort taken to implement a feature is unrelated to that. While the language filters are somewhat related and some of code there might be reusable for tag filters, it is still non-trivial. As with every new feature, it needs to have tests written for it and be code-reviewed to ensure it doesn't break anything, there are performance considerations as every new filter adds more combinations of requests that need to be or can't be cached, then the frontend has to update the user settings schema, add it to the settings page, make it apply in every request, you get the idea. Additionally, features might get stuck waiting behind other larger features or infrastructure changes which were prioritized beforehand.
Now, our API spec is public and there are ways to "hack in" this feature through something like a userscript. You might have to make the exclusion list a more crude manually-edited entry in the script instead of a neat settings page, and you might need to overfetch manga entries to compensate for filtering the entries client-side and fill the page, which might occasionally lead to some jankyness with the rate limits, but hey for a non-official fan-made feature it should be decent enough, and I do encourage anyone interested to try it, I'm sure many of the users here would thank you.