You don't get any strikes for Community Guidelines issues like this, they just shut you down out of the blue. Happened with our previous channel (3.5K+ subs and 3M video views). No warnings, no strikes, nothing. Went to upload a video one day and the channel was gone, with the same message as OP.
YouTube support were insidiously unhelpful and would not give any indication of what we might have done, they just trotted out a boilerplate response.
For the record, the channel was very simple, not monetized and we certainly played no shenanigans with tags and whatnot. There was literally no reason we could see for them doing it.