I don't think it is. At least officially. There's criteria for languages, and Welsh isn't doing so bad. There's plenty of American indigenous languages with 100-1000 speakers in total (most of them elderly), and Welsh's 19% actually translates into the absolute number of around 600,000 speakers...

Or there's a much more informative and informed article: https://www.quora.com/Is-the-Welsh-language-endangered

I suppose there's something to it.... Well, it's not a complete loss. There's books written in Esperanto and that one is a completely invented language, which, I suspect, the majority of speakers learn as adults, or at least not with mother's milk... And then there's Hebrew, which was extinct for at least 1000 years, and now is getting spoken at home.

...So, it's far from thriving, but I would say, relatively speaking it's nothing to worry about.

Also, I think all of its close relatives - the titular Celtic languages - have less than 100,000 speakers (and that's even being generous). I've read recommendations to learn Irish or Scottish Gaelic instead of Welsh... Well, on the one hand, a noble goal, but on the other - I'd really have no way of practicing those outside of moving to Britain.

Meanwhile, I've actually managed to find some sort of prolific Welsh writers.

...I don't know why this... endangered thing makes me... sort of upset. I mean, it kind of has to do with pronunciation. I kind of feel like a thriving, living language has a charm, an accent that is very hard to transmit by teaching it, and impossible by writing about it... And I'm not sure if Welsh hasn't lost that already. It's pronunciation is at times so close to British English that it makes me suspect that there's very little native character left to it.