05AB1E, 8 bytes

Çā<ÅfR+ç

Output as a list of characters.

Try it online or verify all test cases.

Explanation:

Ç        # Convert the (implicit) input-string to a list of codepoint-integers
 ā       # Push a list in the range [1,length] (without popping the list)
  <      # Decrease each by 1 to make the range [0,length)
   Åf    # Map each value to its 0-based n'th Fibonacci number
     R   # Reverse this list
      +  # Add the values in the lists at the same positions together
       ç # Convert it from integers to ASCII characters with these codepoint
         # (after which this list of characters is output implicitly as result)

Vyxal, 51 bits^v2, 6.375 bytes

C:ẏṘ∆F+C

Try it Online!

Bitstring:

010101101100110100101110001100110100000001001011110

Outputs a list of characters.

Explained

C:ẏṘ∆F+C­⁡​‎‎⁡⁠⁡‏‏​⁡⁠⁡‌⁢​‎‎⁡⁠⁢‏⁠‎⁡⁠⁣‏‏​⁡⁠⁡‌⁣​‎‎⁡⁠⁤‏⁠‎⁡⁠⁢⁡‏⁠‎⁡⁠⁢⁢‏‏​⁡⁠⁡‌⁤​‎‎⁡⁠⁢⁣‏‏​⁡⁠⁡‌⁢⁡​‎‎⁡⁠⁢⁤‏‏​⁡⁠⁡‌­
C         # ‎⁡Convert the input to character codes
 :ẏ       # ‎⁢And push the range [0, len(input)), without popping the character codes
   Ṙ∆F    # ‎⁣Generate the fibonacci numbers in reversed order as required
      +   # ‎⁤Add each number to the corresponding char code
       C  # ‎⁢⁡And convert back to characters.
💎

Created with the help of Luminespire.

def reverse_fibonacci_cipher(input_str: str) -> str:
    # Calculate the length of the input string
    n = len(input_str)
    
    # Generate the first n Fibonacci numbers
    fib = [0, 1]
    for i in range(2, n):
        fib.append(fib[-1] + fib[-2])
    
    # Reverse the Fibonacci sequence
    fib = fib[::-1]
    
    # Encode the input string using the reversed Fibonacci sequence
    encoded_chars = [chr(ord(input_str[i]) + fib[i]) for i in range(n)]
    
    # Join the encoded characters to form the encoded string
    encoded_str = ''.join(encoded_chars)
    
    return encoded_str

# Test cases
print(reverse_fibonacci_cipher("Code"))  # Output: "Epee"
print(reverse_fibonacci_cipher("Hello"))  # Output: "Jfnnp"