Tbh I think the display names as they currently work are dangerous. What would prevent me from setting my display name to nutomic@lemmy.ml? Most front-ends would not show that I’m actually not you at first glance.
Edit: Misread the question as being about display names. Full unicode support for usernames would be even more dangerous, basically opening the door to unicode homograph spoofing on that level too.
Yes, that’s what this check (and the later introduced changes) are meant for. On the reasoning why this configuration is not desired I can’t comment.
The check giving this error is pretty old, so it seems that you got it to work was not intentional: https://github.com/LemmyNet/lemmy/blame/63d3759c481ff2d7594d391ae86e881e2aeca56d/crates/api_common/src/utils.rs#L539
As far as I can see, 0.18 and its release candidates only added additional checks preventing such an instance to be set up in the first place.
I’m not sure what exactly you’re asking, but I’ll assume it’s not the standard “I can’t find this remote community” question, as your fourth search result is from the exact community you’re searching for… and has a direct clickable link to the community as federated to your instance (in green).
If you want a direct link as a search result to the community, search for it’s URL on its home instance, e.g. here https://lemmy.ml/c/lemmy_support
You can get this via the link under the community header on the left, where it says “Lemmy Support” and underneath has the hyperlink “!lemmy_support@lemmy.ml”. Click that link or copy its URL.
Which front end would show both names? On (mobile and desktop) web and in Jerboa I only see the display name if it’s set. And the only limitation seems to be that a leading @ is not allowed.