Slack for online communities. Cant we do better? What about err.. Forums?

Slack is being used a lot as a platform for online communities to interact. From meetups and conferences groups to specialized programming languages or tools.
Right now I think I might be in 10 or so different Slack groups from PHP to Kubernetes.
Slack might be a great tool for internal communication in a company (and even this is debatable), but for communities, I see a couple of problems:

it’s not open. You need to find an invite link somewhere to join. The content is not indexable by search engines.
The content is ephemeral. Most of the communities I know have the free slack plan which limits the number of messages you can have. When you reach that limit, the older messages will start to disappear.
Its hard to find useful information afterward unless you are actively involved in the conversation. No one will try to find something useful in a channel with 1000+ unread messages.
You need to create an account in every single slack group you want to join and it’s a pain to find all the groups you belong.
It lacks moderation tools

Discord is getting popular as Slack alternative. At least you only need one account, but it´s a platform more optimized for gamers, not for tech communities or other kinds.
So I was thinking, online communities are not really a new thing. The quantity and popularity yes, but I remember belonging to a couple in the early 2000s, mostly related to games. And how do we communicated back then? Forums!!
Let’s compare Slack with that very old thing called Forums:

Slack is closed, Forums can be public or private.
Slack have channels, forums have sub-forums and topics.
Slack has threads and messages, Forums has threads and messages.
Slack allows you to send direct messages to someone. Forums have private messages.
Slack is very limited in terms of moderation tools. Forums have advanced moderation tools.
Slack have emojis. Forums have emojis and custom signatures!!

Forums seems a pretty decent alternative to Slack for a community, but it seems nobody uses forums anymore? what was the last time you visit one? For technology related stuff, I really can’t remember. Reddit is probably the closest I can think of.
Its not like there aren´t modern forum platforms like Discourse, Flarum or NodeBB.
Maybe we need a free “Forum as a Service" platform?
I don’t know if forums are the answer to all the problems, Slack for sure isn´t. There are definitely lots of work to do in the area of tooling for online communities. I guess the perfect tool could be a mix of Forums, Messaging Platforms, and Social Networks.
Spectrum looks interesting. Let’s see how it evolves after being bought by GitHub.
What’s your thoughts about this?