Gary Illyes, the guy who handles all of this stuff at Google has said on many occasions that subfolder has no benefit over subdomain. In his keynote at Pubcon 2018 he went to far so say that Google doesn’t care.
Yet still we regularly get posts here from users insisting they need subfolder because they read it somewhere on a blog, or have some anecdotal experience which contradicts expert opinion.
That’s the problem here, even after the experts who manage these systems provide definitive answers, people still believe their opinions matter more.
Add the additional technical burden of running a subfolder install of Discourse, and I’m truly baffled why anyone would do it.
Looking at true SEO signals, subfolder actually risks an SEO penalty. TTFB and page load/rendering is a factor for page rankings. Subfolder installations require an extra proxy step between client and server, which introduces additional latency. Hopefully I don’t need to connect the dots there.