I understand this character is forbidden here, but we have no warning of any kind if we add it.
If you add * to markdown_linkify_tlds, there is no issue at first, but if you try to create a topic for example, even without starting to write you’ll have a bunch of javascript errors; the forum will be half-broken. You’ll be able to browse it but not create posts.
Tested on 2.9.0.beta2 4be0e86f1e
Suggestion: prevent the admin (or allow them? there are so much valid domain extensions nowadays…) of adding the * character in markdown_linkify_tlds site setting.