How to take all but the last element in a sequence using LINQ?

The Enumerable.SkipLast(IEnumerable<TSource>, Int32) method was added in .NET Standard 2.1. It does exactly what you want.

IEnumerable<int> sequence = GetSequenceFromExpensiveSource();

var allExceptLast = sequence.SkipLast(1);

From https://learn.microsoft.com/en-us/dotnet/api/system.linq.enumerable.skiplast

Returns a new enumerable collection that contains the elements from source with the last count elements of the source collection omitted.