I wouldn't say this a "Mirrorless" tool per-say, except, the fact that you are seeing the image in your viewfinder, It's always been possible with liveview as others have said. Or simply checking your picture after taking it, to see exposure and contrasts at the time.
Having said that.
I've been taking pictures with Black and white jepgs for years, I always shoot RAW+, but I like to have a preset Black and White conversion jpeg setting. I have agree, seeing the picture in black and white at the time you are taking it is much better than snapping away, and just hoping to get a conversion later in PP or deciding that a picture you have taken, just doesn't do it in colour and then try B&W conversion as an afterthought. I find seeing my picture in Black and White, as I'm taking it, at that time, I am fine tuning the exposure and composition, in real time, for me, it is far better, plus you see shapes and contrasts you simply can miss if you only shoot raw, and only ever preview the colour preview they give.
Here's such a sample, I was taking photos with a number of people on a bridge, we were all talking about producing a Black and white photo, We were all looking at the same bridge, all walking around on the same bridge, but I was the only person looking a a black and white preview of my photos, and whilst doing that, I saw an opportunity, to create a picture that nobody else saw.
This is the jepg, directly from the camera, I did play around on the RAW file doing a conversion, but , in the end, this was already great.
All, my cameras have a RAW + Black & White jpeg User mode saved on them, even my Q, for when I decide, I will be taking a Black and White photo. It's just another tool, why not use it?