Email doesn't do this just fine.
If a new employee joins your group, how are they going to search your inbox for records of technical discussions that they never were part of?
By "that stuff", do you mean "technical communications you have had with colleagues", such as what Slack or email might otherwise be used for? That is what the GP was referring to.
It's hard to imagine using a wiki for that, but if that's how you do things I guess it's fine. As long as it doesn't rely on people copying their email discussions to wikis manually. Not sure how you'd be alerted by an important wiki update that you need to respond to, though.
Let's use a concrete example. A new engineer joins your company and needs to set up a dev environment. This environment has changed many times over the years. Do you point them to Slack, Email, a Wiki, or a shared Google doc? In any sane company, it is one of the latter.
"technical conversations I have had with my colleagues where I can go to recover details I might not remember from a month ago. Then again email also does this just fine, and doesn't cost as much."
For static instructions like setting up a dev environment? Sure a wiki is perfect. But I don't use a wiki for technical conversations like "Hey icedchai, I'm getting an error message that the COM port is not detected and I remember you said something about solving it in yesterday's standup. Can you point me to where the fix was?"
That message is better suited for slack or email than a wiki. (Your solution might be in the wiki but I'm contacting you because it wasn't easy to find, or searching for the error message didn't turn it up).
But if I use email, then when we hire a new employee a month later who gets the same error, your answer will be in my records but not theirs.
Another poster suggested a mailing list, that is a good option that makes email viable for this.
If you use a Wiki for everything, no. However, that is unlikely. You'll have to search more than one place anyway. Making Slack your primary store of institutional knowledge is unwise. Chats are full of garbage, random discussions, old / bad decisions, etc.
No guarantee that Atlassian Wikis will last forever either, but I'm certain that's the wiki 90% will be thinking of when "use a wiki" is suggested.
Backing up your knowledge store is a very separate task from your day-to-day use of it. It's just as possible to back-up your company's slack messages as it is to back-up the company's wiki.