Yeah I did one like that. I did hear back from them a few weeks ago but to be honest I forgot to put the update on here- the response I got from them kind of annoyed me at the time because it’s your typical ‘we stand by our decision but won’t tell you why’ crap 
Someone mentioned above that it could be crypto related. I know i made 1 deposit to coinbase (£200) a while back so maybe it was that.
But it seems ridiculous that 1 deposit would result in a closure of an account. If it’s such an issue surely an email simply saying “don’t do it again” would suffice.
My account and OPs accounts are going to be closed seemingly, but someone from Monzo should really let ALL their customers know not to do crypto deposits otherwise things like this are going to keep happening