More similar tips (from
https://webmasters.stackexchange.com/questions/3496/do-subdomains-help-hurt-seo):

> Subdomains can be just as good for SEO as subdirectories, as long as
> Google knows they are part of the same website. To make sure they look
> like they are the same site to Google:
>
> Link between the subdomain and the main site in the navigation menu,
> and visa-versa.
> Put the same copyright statement on the bottom of each
> Link each to the same terms of service, privacy policy, and contact page
> Use the same logo on each
> Use the same look and feel: colors, fonts, and layout
> Add the subdomain to the same account in Google Search Console
> Use the same Google Analytics and adversiting accounts (such as
> AdSense) on each
Also some speculation on penalties (from the same page):

> Primary domains can pass a penalty to subdomains. Subdomains can’t
> pass a penalty to a main domain unless the main domain holds a clear
> relation to the subdomain.
Most of the generic SEO advice around the net seems to be like this:

> My personal preference on subdomains vs. subdirectories is that I
> usually prefer the convenience of subdirectories for most of my
> content. A subdomain can be useful to separate out content that is
> completely different.
I.e. it boils down to convenience for them. These sites don't have to
think about longevity and consistency because they are made by a single
person or a single company.

Scheme.org should be designed to be ready for mutually non-communicating
groups to maintain different parts of the site without a hitch, if it
comes down to that. And so that the site administration rules are easy
to understand for people we don't know, and with whom we are not
personally in contact, based on a written document.

Likewise, a commercial company or private individual doesn't have to
worry as much about a blog update messing up the site front page. We
have to worry about things like that. For maximum fault tolerance,
ideally our front page would eventually be on its own web server.