Hi, @kamilchlebek, and welcome to our Netlify community site.
You are correct that special characters are not handled properly in build webhooks. We have an open issue filed for this behavior and we’ll update this topic if/when the issue is resolved.
If other people find this issue and community topic, please the original post above to indicate your interest is seeing this resolved.
I walked down the same path as @kamilchlebek trying to get Netlify to play nice with the new version of Heroku’s review apps. For now, my team is gonna have to do something like initials--branch-name instead of our usual initials/branch-name. Would love to see this fixed.