Sorry for not researching myself (I will if nobody else knows off-hand), but are discourse forums being indexed or archived by crawlers? Can it be controlled?
In particular I want to know if I should expect what we write here to still exist, say 15 years from now, on things like the wayback machine (a very useful site, but I do not know what their archiving rules are).
I don’t know about Discourse, or this particular instance, but my rule of thumb: Anything posted on the internet, is already public.
It might be a needle in a very big haystack, but it’s out there.
Yes, this site is actively being crawled and indexed.
I have no idea when and how it will get to the Wayback Machine (I can’t control that) but it’s definitely already indexed and searchable by Google. And probably cached there.
That’s one of the reasons I prefer stuff like this to stuff like Discord (and IRC in most cases). You can actually find things. 
That’s a pity, since it means I will have to do a little more filtering on things that I write with this identity…
It is not a big secret, and I do not mind if you know my real details (Fab already knows, since I used Paypal, and for people who personally know me it would already not be hard to guess) but the idea behind my little experiment was to create some barriers between different parts of my online activity (like Firefox Containers).
Not a big spoke in the wheels of surveillance economy, but may prevent some future nuisances (like “haiders”, at least unsophisticated ones, who knows…), from reaching my real-life family and friends. I have no need to lead a public life, so no reason to make it too easy for them 
I understand this. But I think, overall, it is better for the discourse if things can be found later. One of the things I did not like in Discord was the way people kept repeating themselves over and over again.
And I generally also like it when there’s at least some accountability. There is a very good reason I do everything I do under my real name and have done so on the internet for over 20 years now. I also place a lot more trust in people who do the same.
No worries, will find a way to make the most of it.
If I can help, let me know. We have ways of having user groups and we can make forums that only these groups can see. If a forum isn’t public, it can’t be crawled or indexed.
And if a user has more than one account, that’s between them and their other personalities…