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Sorry @Anonymous (and welcome back!! I missed ya! 
) but I don't believe it will be necessary an Android update to get a new YouTube client version. The app is in the Google Play Store (same release of ours), Google just needs to update the bleeding thing in there and then it will roll out to all Android TV.. That will be done as soon as the awful deal with Samsung expires, I suppose. Apparently Samsung has got half of a monopoly on HDR by streaming. A couple of Italian streaming services support HDR only on Samsung televisions as well. Sorry for the moaning but this kind of things drive me wild.
Instead, @fredwerk, I would also check that the media players you're using do support HDR. I am not sure how HDR works at system level in Windows 10 (actually it's totally new to me). But if you switch the whole Windows to HDR, and the players do not support HDR, I believe you just end up playing the videos in SDR with the tv forced to HDR. I think your best bet is to try with WIndows Media Player. I mean, if Windows supports HDR, so should do its stock apps do. Or maybe it is a matter of installing the right Codecs. It needs a bit more of investigation. Fortunately I am not an Windows expert, I use a Mac. 
@Jecht_Sin wrote:
Sorry @Anonymous (and welcome back!! I missed ya!
Awe thanks @Jecht_Sin 
You're probably right about the YouTube app thing though, I forgot about what stage the update cycle was on. Although as its one of the built in apps it could work both ways I guess. I didnt know that Samsung has a deal on the HDR streaming - I dont suppose you have a link to that by any chance?
This is certainly what happened with a couple of streaming apps for the UK a couple of years back (ITV Player and ALL4) - they purchased the rights for sole use for a 12 month period at that time. Samsung seems to have a far larger TV budget than Sony. But this purchasing of sole use off rights is infuriating.
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You're very welcome, @Anonymous! 
So, about the YouTube app: my understanding is that the situation is quite different from the Amazon Video app, for example. The latter should receive an update only through an Android TV (firmware. But I hate to use that term, it isn't correct) update (and so I have been told by the useless Amazon support). While being YouTube in Google Play, there are little reasons for the app not to update as soon as a new version is in there. I think it would be easy to check with TVs still running Lollipop. If they have the option to uninstall the update, one can uninstall it and then see if it gets udated from the store (like Google Movie & C. Those apps do get updated via the store. Not sure if Google Movie supports HDR either, though).
About the Samsung deal.. I don't have any proof, but sure it isn't a concidence that HDR for YouTube only runs on Samsung and Chromecast, that Infinity plays HDR content only on Samsung, Mediaset Premium Play does the same. Also, regarding YouTube, the app for PS4 Pro, which has got quite few updates (4K support included), as far as I am aware does not currently support HDR (at least there is nothing in the update history - I have the standard PS4 so I can't check). And I strongly doubt the PS4 doesn't support the profile vp9.2. I don't know about the Xbone One S, but I doubt it's supported either (also I checked in Google right now, all artciles/posts say it isn't supported).
Then I don't know if it is related to this, but Infinity claimed that HDR would have started rolling out first on Samsung tvs for three months (meaning that after this time more devices were going to support it). That was November 2016. And still now only Samsung does have it. I hope, if there was really any deal, that it hasn't been even extended! And with all partners, of course..
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