I don’t think they are shadow banning – I have no evidence – but they are effectively doing so by delaying the publication of a comment.
After 6-24 hours it appears well down in the comments and often when other articles have been published. So most readers won’t come across it unless they bother reading every comment in a older article with hundreds of comments.
It is therefore easy the comment doesn’t get seen at all (except by their Guardian Angel mods).
I did many naive moons ago helpfully suggest that there was no problem in having a comment in ‘mod’ but when it was published it should be posted at the front with a time stamp that it was let out of jail – so that it may be seen as it was written, as a instant opinion to be discussed with other readers who have also just come across the article. But that fell on deaf ears and they ratcheted down in it further as I found more inventive ways of bypassing their attempts at attempted ‘shadow banning’ .
They have been forced to show their hand.