SharePoint Online Save as Site Template option gone?

About the situation that the Team Site cannot be saved as templates after activating SharePoint Server Publishing Infrastructure, we can re-enable the Save site as template feature in SharePoint Designer 2013.

For detailed steps, you can refer to the information below.

1. Open the site in SharePoint Designer 2013.

2. Click Site Options in the top ribbon->SaveSiteAsTemplateEnabled->Modify->change False to True, then click OK->OK.

3. Click Save as Template in the top ribbon, type a name, select Include Content, and click OK.

4. It may take a while to work on the progress. Go to the Solutions Gallery to check the template.

Note: Some apps or certain content added to the Team Site may not be allowed when you want to save Team Site as template. If you want to continue to use this method, it's required to remove some content or functionality until the process can be going on.

For more details refer this link: https://community.office365.com/en-us/f/154/t/164502