Is this still happening?
Look, I am not an authority on the subject, but I have a keen amateur interest in languages in general. In a past life, I spent more hours than I care to think about closely reading pronunciation guides of varying qualities and listening to recordings of native pronunciation in multiple languages, in efforts to produce a decent accent by the time of my A-level orals!
Can we just take it as read that almost everyone on this thread is probably pronouncing Siân correctly, other than the sigh-annes and the see-annes?
Fact is, recording pronunciation in a written form that is 100% accurate and also 100% legible to any reader, whatever their own linguistic background, is a professional speciality. No nasturiums need to be thrown at anyone in this thread for not managing it. All the votes for shaans, sharns and shahns are people trying to describe the same name. It is true that tways are more location-specific that others and would be actively misleading to a reader raised elsewhere.
We can also take it for granted that any Welsh MNer who votes for shan is also going to be pronouncing it correctly. Again, exactly like the controversial sharn, their method is grounded in their linguistic background. It probably works for anyone local to them, just like sharn works for a Londoner explaining to another Londoner that it isn't bloody sigh-anne. Shan would be actively unhelpful in the latter situation, exactly as writing sharn is unhelpful to any Scots MNer with a rhotic accent.
I have absolutely no idea what is going on with shan advocates who are not Welsh. Maybe they're pronouncing it correctly, maybe they're not.