EqualsIgnoreCaseAndDiacritics.cs

using System;
using System.Globalization;
using System.Linq;
using System.Text;

EqualsIgnoreCaseAndDiacritics("héLloṩ", "hellos"); // true

static string RemoveDiacritics(string str) => 
	str is null ? null :
		string.Concat(str
			.Normalize(NormalizationForm.FormD)
			.Where(ch => CharUnicodeInfo.GetUnicodeCategory(ch) != UnicodeCategory.NonSpacingMark));

static bool EqualsIgnoreCaseAndDiacritics(string a, string b) =>
	string.Equals(RemoveDiacritics(a), RemoveDiacritics(b), StringComparison.OrdinalIgnoreCase);