Creating a SharePoint Online modern team site without an Office 365 Group

Thanks for the link ronalg . After further research, it's not documented and all the examples found are using the communications site template, but we tested successfully with Flow using the endpoint: https://tenant.sharepoint.com/_api/SPSiteManager/Create and the webtemplate STS#3.

This can create Modern Team site collection without O365 Group. Sample body:

{
"request": {
"__metadata": {
"type": "Microsoft.SharePoint.Portal.SPSiteCreationRequest"
},
"WebTemplate": "STS#3",
"Title": "showcasetest",
"Url": "https://tenant.sharepoint.com/sites/siteTitle",
"Description": "test",
"Classification": "",
"SiteDesignId": "00000000-0000-0000-0000-000000000000",
"Lcid": 1033,
"ShareByEmailEnabled": false,
"WebTemplateExtensionId": "00000000-0000-0000-0000-000000000000",
"Owner": "i:0#.f|membership|user@something.com",
"HubSiteId": "00000000-0000-0000-0000-000000000000"
}
}

References: 

1. https://sharepoint.stackexchange.com/questions/259917/create-new-site-collection-with-rest-api
2. https://simonagren.github.io/sites-sitedesign-rest/
3. https://github.com/SharePoint/sp-dev-docs/issues/2148