Many non-Twitter guides to Twitter cards recommend using a twitter:url tag. But I can find no reference on dev.twitter.com such as in the Twitter Cards documentation. Is it a valid meta tag? Is it deprecated?
It has been deprecated. Thanks for catching this; if you see particular entries, please be sure to tell us here and we’ll update the docs.
Sorry for the confusion!
And it’s been deprecated in favour of what?
The URL is implicit from the tags on which the pages sit.