Thinking back to the good old days, my main forms of discovery were IRC, Usenet (with its underlying structure it made it easy to dig into weird/different niches at will)...
A few things were consistent about these mechanisms: Each one allowed you to start in a specific niche/community, explore outside of it and become entrenched in wider/more diverse communities/interests.
They had some basic structure (channels on IRC, top level groups on Usenet), some form of curation/moderation (channel operators, news server admins who could white/blacklist content, rules/guidelines about cross-posting, some proactive moves against spam/off topic trolling), but ultimately were "open."
Anyone could get their foot in with some basic software, anyone could add their voice, and engage either with a group of people or one-on-one.