its not hierarchically wrong; services never required their own subdomain - example.com is just as valid a host as www.example.com

sure it is helpful for organization, but when was the last time you used ssh.example.com?

Don't get me wrong - it's in no way illegal to use A/AAAA-pointers in your zone root.

RFC 1034[0] however, argues for a tree-based structure and lays emphasis on the value of branching. It being 'wrong' might have been too strong a description, how about it being 'improper'?

When I've seen SSH access being offered as a network service and not as a means to administrate other services on a network it's often been under names such as 'shell.example.com' - not exactly your 'ssh' label but an approximation.

0. https://tools.ietf.org/html/rfc1034

Naturally, Example LLC will serve html on port 80, because that's Hypertext. And while that does not have to redirect to the www homepage, there is hardly any other reasonable alternative I could think of. Likewise, I think I read here, a bare tld domain could resolve, too, I just don't remember what it would or should resolve to.