Presumably this is a response to the Online Safety Act.

However it doesn't make much sense, because Imgur no longer hosts sexually-explicit content due to pressure from advertisers, and there has been no communication from them to suggest that they have had any trouble due to of the content that people warned may get caught up in the OSA (which, while a real risk, would be a lengthy court battle)

For me this is an inevitable result of the OSA but Imgur also likely bears some responsibility. If 4chan is still available in the UK, there was no need for Imgur to go dark, at least not at this stage.