https://learn.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-13#implicit-index-access

The implicit "from the end" index operator, ^, is now allowed in an object initializer expression.
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The preceding example creates an array that counts down from 9 to 0. In versions before C# 13, the ^ operator can't be used in an object initializer. You need to index the elements from the front.