No, it was definitely possible to meet new people. The propagation of these closed spaces worsens the loneliness epidemic, which is way worse than at any point in history.

The closed space of a group chat with friends worsens the loneliness epidemic? And arguing with random strangers on Reddit alleviates it? It's very much the opposite. Big, open and often anonymous social spaces are terrible at making you feel part of a group. Spaces with few people who you actually know and like and can depend on are key to not feeling lonely, and yes they tend to be closed.

Exactly, the reason that people feel so alone is because of void "interactions" on social media.

Closed doesn't mean undiscoverable. The biggest closed group chat I'm in is the Telegram channel for a podcast I listen to, and it's plugged in every episode. Discord has issues, but group chats there are still mostly fed by public activity like subreddits or YouTube channels.

Closed just means you can't open a page and read the full history like a forum. It doesn't mean it's only knowable to people currently inside and people they invite.

Maybe a social network where you can only have (or follow) a maximum of 30 (or similar) people?

I think a social network that combines something like Discord and old Facebook timelines would be interesting. Basically group chats would become first class citizens, and the platform should help you discover group chats that your friends are in. That was the missing piece from Facebook messenger group chats. They tried to introduce "Rooms" but it failed since other platforms had already taken over that space. Maybe it is still too late.