In short, we can talk about its existence, we cannot mention names. Or can names be said without there being a direct recommendation?
For example: "Yesterday mixxptoabcd was closed by the US authorities, follow the news..."
I guess it's yes we can talk about mixers but we do not allow to mention their sites, name of service, links, or guide them directly by saying "Search it using this keyword" It is considered a "direct recommendation" you are guiding him/her to a mixer.
About the news with the mentioned name of the mixer service, I think it should be fine because you redirecting people to the news and it's service is already closed.