Change 175750 had a related patch set uploaded (by Legoktm):
Display username for AntiSpoof conflicts

https://gerrit.wikimedia.org/r/175750

Change 175750 merged by jenkins-bot:
Display username for AntiSpoof conflicts

https://gerrit.wikimedia.org/r/175750

Shouldn't this show more than one conflicting name at the same time?

This is just an example (almost like a real case, just the characters are different):
I'm going to rename "ödsgdksghdsn" to "ABC"
I get "The chosen username is too similar to existing usernames: A bc."

When I look at the CentralAuth I can find that there are also other usernames like "ABC", for example "Abc".
Why it doesn't mention all of those conflicting usernames?

In T75573#1234657, @Stryn wrote:

Shouldn't this show more than one conflicting name at the same time?

This is just an example (almost like a real case, just the characters are different):
I'm going to rename "ödsgdksghdsn" to "ABC"
I get "The chosen username is too similar to existing usernames: A bc."

When I look at the CentralAuth I can find that there are also other usernames like "ABC", for example "Abc".
Why it doesn't mention all of those conflicting usernames?

It does support showing multiple usernames and it seems to work. If it doesn't show a similar username, it means AntiSpoof thinks that the username is not similar.