Microsoft launched its communication tool for organizations, Teams, over a year ago as part of its Office 365 offering and it's been working to push the service forward since then, even replacing Skype for Business with it. More recently, the company began testing a progressive web app (PWA) for Teams in the Microsoft Store which has since become publicly available, allowing Windows 10 S users to access the service more easily without upgrading to Windows 10 Pro or using a web browser.
Earlier this year, evidence found in a developer preview suggested that the company might be planning a freemium model for the service, with little more to go on aside from an indication that the free version might impose storage limits.
Now, more evidence seems to point in the same direction, as Twitter user WalkingCat discovered some official documentation from Microsoft regarding a yet-unannounced free tier for Microsoft Teams. This includes an End User Notice which repeatedly references "Microsoft Teams (Free)".
Microsoft Teams (free) is intended for use by organizations, such as small businesses or groups of coworkers. If you were invited to join a group in Microsoft Teams (free), the group you are joining in the Microsoft Teams (free) instance will be considered your organization for the purposes of this notice, and the person administering the group will be considered the administrator of your account in Microsoft Teams (free).
The newly discovered references certainly make it seem more likely that a free version is indeed coming in the near future, but there's still not a lot of information regarding the capabilities and limitations of the non-paid tier, although it's probably a matter of time until Microsoft officially announces the release along with those details.