First, we should recognize this is in the gray area between censorship and not hosting offensive material, and people will reasonably disagree with one another. I therefore expect that no policy will be a perfect solution. Thus...
Summary: I recommend:
- respecting the original author's decision to link or not link,
- optionally adding an unobtrusive and impartial-sounding adjective such as "...the Shia-critical site [link]..." if relevant, and
- adding a pertinent snippet from the site (to reduce the need to access the site).
Monica Cellio, diamond moderator at Mi Yodeya (i.e., Judaism.SE), proposed removing links to actively hostile sites, which seems reasonable to me.
Should we forbid linking to anti-any-religious groups sites?
Forbidding these links goes against the sectarian nature of the site (this is a question and answer site; it's neither pro-XYZ nor anti-XYZ). While it's against StackExchange policy to host offensive material (see: Stack Exchange Network Terms of Service), this is counterbalanced by StackExchange's general attitude towards censorship. In this situation, Islam.SE doesn't host the content of those links.
These links provide context. It may be that visiting the site is needed to understand a post, and not providing the link is an obstacle. Providing a pertinent snippet from the site, thereby giving a self-contained answer/question, reduces the need for someone to click on the link. (And many anti-[anything] sites are prone to link rot.)
It's going to be a judgement call (i.e., a matter of opinion). In these cases, we should respect the original author's decision; they best understand whether or not a link is needed to understand their post. Otherwise, we risk arguments, edit wars, and antipathy towards the site. (There's going to be exceptions, e.g. if the link encourages violence.)
We can optionally add some adjective e.g. "...the Shia-critical site [link]...", something that's both unobtrusive and is not itself offensive. Then reader can make an informed decision as to whether or not they want to click the link.
A similar question was asked on meta.Judaism.SE Censoring comments, and it was argued against for the following reasons: (a) an implementation nightmare, (b) a bureaucratic nightmare, (c) unnecessary, (d) bad practice for this site.
I asked the question at meta.SE: Should we edit out useful links to likely offensive websites? Hopefully, we can get a better perspective from more experienced sites and people more familiarity with StackExchange policy. The question got this response from Monica Cellio:
We almost always do this in the case of a site that is actively hostile, abusive, or fraudulent (as opposed to just wrong). We feel no obligation to provide visibility and promotion for hate sites, for example.
For sites that aren't like that but are more mildly problematic, often we keep links but make sure the nature of the link is clear without clicking.
This is somewhat consistent with goldPseudo's answer.