I haven't personally applied for them this time around, however they are one of the firms I was majorly considering. Unethical might be the wrong word to use here. But, to me at least, even if you find candidates that you want to accept onto your VS I think you should still have the responsibility to review all other applications - since applications take lots of time and effort. And this is especially the case if you claim to be reviewing applications non-rolling.
On top of that, if you receive a higher number of applicants than you can actually commit to reviewing then the application window should be shortened (I'm pretty sure other firms did this). That's just my opinion, I just feel awful for those who spent hours or days researching the firm.
I think the point of claiming to be non-rolling but then cutting off reviewing applications is very fair. I imagine from the grad recruitment perspective however the last year has just been unprecedented, and the usual targets they would build in have to be thrown out the window. Otherwise they would have to hire more recruiters, which makes no financial sense from their perspective if they still want to hire the same numbers.
It massively sucks, however it's the case equally in almost all competitive sectors in London. Regardless of how much a specific job seems your dream job, anything in London gets so many applications it's best not even considering it once it's sent. For the vast majority of grad level jobs I have applied for in London over the years, I haven't even heard back. It works when you accept that a lot of your applications will never even be read. It doesn't mean yours didn't meet the benchmark, but it makes no sense from a business perspective to interview say 100 people for a grad charity job when if they interview 5-10 they'll likely find someone they like. Rest straight in the bin. In comparison to most sectors, law firms spend ridiculous levels of time and money on picking their candidates.
Applying to grad-level jobs in London SUCKS. You send off a tonne of applications, no responses from anything you are qualified for, then randomly get one! Not unique to law--if anything it's less opaque.