No you redirect to the right place. It’s no worse than writing obscene things in a URL fragment (after #) that doesn’t even get sent to the server.

It’s not great but it matters less when the content you get going to that page is so unremarkable. Don’t forget you can do that to any url, even of sites that don’t use optional slugs, if your goal is just vague, evil-by-link-appearance.

What is the actual harm in allowing people to put random text at the end of the url?

Not to mention something similar can be done to any url, e.g. #whatever-you-want or ?_=whatever-you-want

This is not materially different from https://stackoverflow.com/questions/78870603#i-LOVE-genocide or https://stackoverflow.com/questions/78870603?i-LOVE-genocide.