“Search engines keep different metrics for domains than they do for subdomains, so even though Google itself has stated that — from a ranking perspective — content in subdomains and subdirectories is treated roughly equally, it's still recommended that webmasters place link-worthy content like blogs in subfolders rather than subdomains (i.e. www.example.com/blog/ rather than blog.example.com). ”

https://fly.io/articles/one-hostname-to-rule-them-all/