I see from the Sony support site that Google is planning to withdraw certain older YouTube apps, such as the ones on my Sony TV and Blu-ray player, from 20 April 2015. There are lists of the affected equipment on the Sony website.

When the old "big-screen" BBC iPlayer was withdrawn last year, it disappeared from my Blu-ray player, but the TV version kept going, fortunately. No such luck this time.

Unless I've missed others, as is very possible, yours seems to be the only post on Digital Spy about this, which seems surprising. There is much unhappy discussion elsewhere on the internet.
My Sony TV has the excellent, but soon to be withdrawn Google YouTube app and also an app called YouTube for TV, which is dreadful and keeps crashing the TV despite having the latest software. Apparently after 20th April the youTube for TV app will still 'work' - except it doesn't!
I also have a Panasonic Bluray device (purchased 6 months ago though a 2013 model) which has the Google YouTube app but no alternative available. Panasonic, being dreadful at updating even recent equipment software, will presumably just ignore the problem even though YouTube was advertised as a desirable feature of the equipment.
In addition, I note that the Virgin Media YouTube app available on Tivos is currently not working very well and I wonder if it may be related to this matter.
No doubt Google have a commercial reason for this decision. There seem to be ads with every video on the YouTube for TV app, which may be a factor. Perhaps it is to encourage their Chromecast device (which presumably is compatible with YouTube).
Overall very annoying.

To be fair, nothing actually seems to work well on Virgin's Tivo. iPlayer does not. Youtube does not. Netflix is incredibly slow on it. Accessing My Shows takes an age to load up and show you the recordings. Virgin Media Tivo is basically pants.

Thanks for you reply, Hector; I was beginning to feel a bit lonely after 10 weeks with none.
I am particularly annoyed about the YouTube app as I had only fairly recently gone through a lengthy rigmarole fiddling about with YouTube usernames etc. in order to be able to sign in to the app and have my favourites list available. The picture quality and sound were much better than I get on my PC.

Oh well, that's progress, I suppose.

2-3 weeks ago, I bought a Chromecast to be able to continue watching YouTube on my 65 inch Panasonic TV, as YouTube is disappearing from my TV on the 20th April as well.
Ian.

When GOOGLE buys something good they screw it up. It's about time apple or some other company buy it out. Too many copyright laws, and now google takes 55% of your advertising revenue. Anyway I think YouTube finished goodbye YouTube. The new kid on the block is twitch.tv.