Given that Microsoft has abandoned their TFS CmdLets, one alternative is to access VSTS data is through the OData interface using the Invoke-RestMethod.
Before you begin you need to setup Alternate authentication credentials by navigating to https://<yourtenant>.visualstudio.com/_details/security/altcreds. Once these are established, you will likely want to save them to a local PowerShell variable so that they can be reused for each Invoke-RestMethod invocation.
$vstsCredentials = Get-Credential
IMPORTANT: When entering your credentials, be sure to prefix it with your tenant name. For example, if your VSTS URL is <domain>.visualstudio.com and your alternate credential username is “InigoMontoya”, you will want to enter a username of <domain>\InigoMontoya.
Next, you want to invoke the appropriate TFS OData resource URL (see https://tfsodata.visualstudio.com/) corresponding to the type of data you want to access (Projects, WorkItems, Builds, etc). For example, to access the projects associated with your tenant you would use:
$projectsData = Invoke-RestMethod -Uri 'https://tfsodata.visualstudio.com/DefaultCollection/Projects?$format=json' -Credential $vstsCredentials
To view the results you would dereference the result as follows:
$projectsData.d.results | Select-Object Name,Collection
The result will be a list of projects and their corresponding collection URL (which are likely all the same on VSTS).