It is still possible to add an external link but to get to the site you would have to copy/paste it into your browser manually. A small risk is still there but it is not as serious as I initially thought.
Yes, it is not too serious because dangerous links can not directly trap users by clicking on subject titles. Nevertheless, there are still potential shady behaviours from bad guys to change subject titles for their own interests, such as FUDs, or trolls. I know that there are good usages like what @Jet Cash usually does and mentioned in previous posts. Generally, I still think it should be disabled. Subject titles should only be edited by OPs, and after each changes, all subject titles inside one topic should be changed automatically to newest one. It makes more sense for bounty topics. When campaigns end, all subject titles (not only the OP and last one) in the bounty topics should be displayed with [FIN] or whatever abbreviation managers use to announce campaigns actually end.