you see a lot of it here from the 2010s. ex: https://news.ycombinator.com/item?id=1176438
But I just ran into a comment on reddit saying that modern Google does not make a distinction anymore. If that's true it would be a hell of a lot easier for me. I could setup the subdomain proxying via cloudflare super easily. If I had to do a subdirectory I would either have to setup cloudflare workers or do some much more complicated reverse proxying
https://www.searchenginejournal.com/google-treats-subdomains-subdirectories-john-mueller-says/254687/