Getting that message in at least a couple of threads. Is it self censorship by Forum Mods, or something external ?
Its external, it is the linked websites in question blocking your request.
There are two possible reasons this could be happening.
1. Like others have suggested it could be due to you using a VPN, proxy, or tor relay. Some sites make efforts to block connections from such services. Or from traffic not coming from residential IP addresses.
It could also be the case that the website restricts traffic from certain countries due to licensing, legalities, or whatever else.
2. Or it could be that the site only fails to load in the message link preview on these forums. Websites can determine where your coming from and where their links are being used. (Using the refer property).
If they blocked traffic being referred from SS it will fail to load a link preview potentially with a 403 error.
You can get around this issue by opening the link in a new tab, or copying and pasting the link into a new tab or window.
You can also change settings in your browser to not send refer data, as so the websites in question do not know where you clicked the link.
I get these errors too, due to my own security measures, I have found my ways around them by taking the above into account.
If you need any help, just ask, or PM.
Edited to add, here is a useful post by
@Jen Erik on disabling browser refer data. The links will still say they are unavailable in the preview on the forum, but ignore it and just click them.
If they still are not loading its the first issue (1) I mentioned.