Russia Today and Sputnik, two large news organizations funded and run by the Russian government, are now being blocked from advertising on Twitter. This is one step the social network is taking to impede Russia's ability to use the platform for political purposes.
Off-boarding advertising from all accounts owned by Russia Today (RT) and Sputnik.
We’re donating all projected earnings (
.9mm) to support external research into the use of Twitter in elections, including use of malicious automation and misinformation. https://t.co/zIxfqqXCZr— jack (@jack) October 26, 2017
It implemented the ban against those sources in particular because Twitter discovered they "attempted to interfere with the  election on behalf of the Russian government." The company has been investigating Russia's use of its services and it's also relying on input from the US intelligence community, which placed blame on the news organizations.
In response to this action, RT's social media head alleged Twitter wanted RT to run advertising based around the election.