They apologize for the confusion that this created for some developers and content creators. They know that the tool has historically been used to check how sites render when links are shared on Twitter; however, the display previews were not always aligned with the display of cards on Twitter across different platforms. The best way to check this is via the app itself.

If the Card Validator shows that it is unable to render your card, this may not mean that the card is not functional. They encourage you to use the Tweet Composer in the Twitter client itself (on Twitter.com, or the Twitter for Android or Twitter for iOS apps) to check out a preview that would accurately reflect the current display of the card on Twitter. To do this, start to create a new Tweet and paste the link into the Tweet Composer - there should be no need to post the Tweet, as the card preview will render automatically if the URL has appropriate card metadata. If the card does not show in Tweet Composer, you may wish to troubleshoot.

The legacy Card Validator page remains accessible for submitting brand new domains, and for using the logs to confirm that cards are accessible or recognised by Twitter on specific URLs - previews will no longer be shown there. Cards data continues to be cached for up to 7 days before being refreshed. The tool can also be used to provide a "hint" to the cards crawler to revisit your site, by adding a unique parameter to the URL, as described in the cards troubleshooting documentation, but in many cases there may be a delay before new metadata is reflected on Twitter itself. We're exploring ways to provide more creative options for displaying content in Tweets in the future

There is an additional guide to checking cards functionality here. The post includes links to the cards documentation (for markup instructions), information about how to check for images that cannot be accessed by Twitter's servers, and diagnosing network and domain issues.