According to the official announcement, starting in October Google will allow registered cryptocurrency exchanges to advertise on its Google Adwords platform, targeting the U.S. and Japanese audiences. The announcement says:

Advertisers will need to be certified with Google for the specific country in which their ads will serve. Advertisers will be able to apply for certification once the policy launches in October.
This decision follows an announcement in March that all crypto-related businesses will be banned from buying ads on Google Adwords, described by industry insiders as unfair and troubling.
To justify its crypto ad ban, Google said that it was protecting its customers from fraudulent offerings, including but not limited to initial coin offerings, cryptocurrency exchanges, cryptocurrency wallets, and cryptocurrency trading advice.
Other tech giants, such as Facebook and Twitter have made similar statements in a wave of crypto advertisement bans earlier this year.
Since then, Facebook has reversed its ad ban for pre-approved cryptocurrency firms, while still maintaining a ban on Initial Coin Offering (ICO) advertisement a move similar to the one made today by Google.
Back in June, during an exclusive interview with Cointelegraph, Wikipedias Jimmy Wales commented on the attempts to regulate the blockchain and crypto industries, saying: You can't ban math. You cant ban blockchain.
In July, Googles co-founder Sergey Brin announced that he is an Ethereum (ETH) miner at the Blockchain Summit in Morocco.