In the long long ago...
In the long long ago, before the great AI wars, SEO was easier. People submitted sitemaps to search engines (there were lots of them back then). And there were these things called directories where people could submit sites - and other people could find them. I'm looking at you Yahoo. Those days are long gone though.
I'm trying to figure out how to generate traffic these days, and it's not easy. Google and Bing are the only places to submit a sitemap anymore. No guarantee they'll choose to index your content though. Directories don't exist - unless you count the thousand different "product launch" sites that want to charge $40 for a backlink. Maybe there's a secret to it, maybe I'll find it. But I kinda miss the long long ago.
I think one of the biggest shifts is that discovery has become much more reputation-driven than submission-driven.
A sitemap tells search engines your pages exist. It doesn't give them a reason to trust or surface them. More and more, distribution seems to come from becoming a source that's worth citing rather than simply making content that's worth indexing.