Siteground provided GrowBig and GoGeek account users with the free wildcard SSL. However, Siteground has updated the feature list of their GrowBig and GoGeek plans by removing the free Wildcard SSL from GlobalSign. It remains available as a paid service.
Please note that Siteground continues offering everyone the unlimited free Let’s Encrypt SSLs with every plan, which is a great service to highlight to customers.
Let’s Encrypt is a free, automated, and open certificate authority (CA) that issues domain-validated security certificates.
You can apply free Let's Encrypt SSLs to all domains under Security > Let's Encrypt in cPanel.
Just for your reference: Bluehost offers free SSLs to their VPS users as well as Shared Hosting users.