Can I install OS X 10.10 on a MBP Mid-2015 I buy today?
Interesting question.

Originally these models shipped with macOS Yosemite 10.10.3. However, my understanding is that current models ship with macOS El Capitan 10.11.2.1

Typically this would mean you can’t install a version of macOS earlier than the version it actually ships with and I suspect that would still be the case here.

However, I don’t recall seeing any firmware updates for this model, so it may be possible to do what you want manually (i.e. you wouldn’t be able to do it from Recovery Mode, you’d have to try booting from an external installer or try using Target Disk Mode).

My inclination would be to say it isn’t going to be possible, but in this rare case it may be.

Hopefully someone else who has tried this can chime in here and give you a more definitive answer. You could also contact Apple and see what they say.

1. Last time I checked (admittedly a few months ago - early this year so well after macOS High Sierra was released) these models were still shipping with 10.11.2. However, I am based in Australia and it could be just a case of how fast production batches are selling. I will check again in a couple of hours and update this answer as soon as I can.

[UPDATE]

I've just had confirmation that models being manufactured now ship directly from the factory with macOS High Sierra installed. My guess is getting one with El Capitan pre-installed is going to be a remote possibility at best, and even then that'd be based on whether your location has older stock.

So, installing an older version of macOS depends entirely on whether there's been a firmware update on that model. Unfortunately, some years back Apple started automatically including firmware updates (if/when required) within macOS updates/upgrades and I'm not aware of any current consolidated list of these updates, so it's not easy to check which devices had firmware updates and what those updates meant. Previously you could refer to this page.

In other words the only way to find out with 100% certainty is to try it, but I'd err on the side of caution and assume this won't be possible. I suppose you could ask Apple if you could return it within the return period on the basis that you can't install Yosemite, but they're likely to tell you up front that it's not going to be possible.

The only way to be 100% sure would be to purchase one of the original ones that hasn't been updated beyond Yosemite, although I suspect that won't be easy to find. You'd also be 99+% sure if you bought an original one that shipped with Yosemite but has since been upgraded. However, if you purchased one brand new today that ships with High Sierra, your odds drop to less than 50/50 in my opinion.