Not-a-duplicate note: YouTube's policies for offline content have changed many times since the questions Download YouTube Videos from Mobile? and Is it possible to pre-download a YouTube video to view it later offline? were asked.
I'm asking specifically about the what is possible and allowed AFTER Google's policies changed with the 2015 introduction of Music Key.
I gather that in 2014 YouTube allowed videos to be stored (temporarily?) for offline viewing. From articles about it I can find online (example, June 2015), this seemed to apply to all videos unless the content provider opted out.
In September 2015 I'm trying to find out what the policy is now. The only up to date official information I can find relates specifically to music videos, specifically for Music Key subscribers. It gives the impression it's (now?) a subscriber-only, music-only feature, but that doesn't seem to match what I read elsewhere. I can't find any up to date information about non-music videos, and I haven't successfully found a video that can be saved for offline use, but I might just be looking in the wrong place.
A lot of the information I've found is confusing. For example, this May 2015 PC Advisor article claims that the feature has been promised but not rolled out yet, while this earlier Dec 2014 India Today article says the feature HAS been rolled out (but might be specific to India?), and this recent August 2015 Android.SE question shows a user using the feature successfully for non-music videos. One answer to that question suggests the policy varies by country - and I can't find any country-by-country guide. I can't see any offline link for the videos that other asker had successfully downloaded.
Here's a Gizmodo article from Sept 2014 which appears to confirm that Sept 2014 launch was India-only. Apparently Indonesia and the Philippines got it too in December 2014, though it sounds like that's only for some promoted content like movie trailers (implied, not explicit). These are the only articles I've found that directly address regional differences.
I tried asking Google directly, but they refused to answer my question at all:
there isn't a support team that deals with free YouTube videos
The closest I can find online to anything representing an official comment from Google is this clearly unofficial reddit post from someone talking as if they represent google (abridged):
Each country we launch a feature like offline videos in is a legal struggle, because culture, copyright law, who owns what copyrights... We'd like to do offline playback everywhere, of course... Offline playback was really important for India, so it got launched there earlier than elsewhere. Keep an eye out, it may show up...
So if it is true it's only in those three Asian countries, that's a plausible explanation for why. But it still doesn't answer what the policy is right now.
I don't want to install non-Play apps, or root, or use a custom ROM, or do anything illegal, I'm just trying to figure out if and how I can legally cache a non-music YouTube video somewhere I have WiFi, and watch it somewhere I don't.