Yes, ads are coming to the Windows 10 Start menu. If you're an Insider on the "Fast Ring" and have downloaded build 10565 then you might have already encountered them.
And it seems these ads aren't something Microsoft is running up the flagpole just on the Insider builds. This is a feature that's likely to be in the Windows 10 Threshold 2 update that will be pushed out to users sometime in November.
"We will continue to offer Windows Ads in Apps on Windows 10. Beyond that, we do not currently have plans for advertising in Windows 10," a Microsoft spokesperson told The Inquirer. "Lock and Start content is programmed by Microsoft to help customers learn and discover new features and apps to enhance their Windows 10 experience; app publishers are not paying to be featured."
The ads - which are labeled "suggested" - are for apps in the Windows Store, and are prominently featured in the "Most Used" section. If you're not bothered by this then don't worry about it, but if you want them gone, here's how to make that happen.
Right-click on the ad and select "Don't show this suggestion" if you don't want to see that particular ad again, or "Turn off all suggestions" if you want the ads to go away altogether.
Another way to make these "suggestions" go away is to go to Settings > Personalization > Start and then flip the "Occasionally show suggestions in Start" switch to the off position.
I expect people will fall into one of two camps with regards to these ads. Some will be outraged at the liberties being taken with "their" Start menu, others won't care and might find the extra discoverability for apps handy.
As for me, as long as there exists a way to turn them off - and I still remember the days of seizure-inducing ads in applications such as MSN Messenger which couldn't be turned off without some ugly hacking of the executable - then I'm honestly not that fussed.