Update: Response from @ukhomeoffice PermSec to my complaint:

"I agree the phrase you quote should not have been used on an official government channel. I have made clear to the team this post should not be used again from Home Office accounts or anywhere else by civil servants."

This was a very clear breach of the Government Communications Service Propriety Standards. The prompt reaction of the Permanent Secretary is very welcome. I hope comms teams/Press Offices in Home Office & other Depts take note, & this unacceptable behaviour is not repeated.

[A number of people have pointed out that the post has not been deleted. I would suggest that anyone who thinks that it should, given the PS' clear view that the wording was unacceptable, complains direct to HO here: https://www.gov.uk/government/organisations/home-office/about/complaints-procedure ]