I've almost finished this hard sudoku, but I cannot finish it without guessing or making multiple blind steps to see if I'm wrong or not. Even https://www.sudoku-solutions.com/ gives up and says "there is no hint available". I've tried to apply as many solving techniques as possible, but to no avail. Can anyone help?
If F1 is a 2, then
G1 is "not 2", G9 must be 2, and E9 cannot be a 2.
On the other hand, if F1 is not a 2, then
E2 must be a 2, so E9 cannot be 2.
E9 cannot be 2.
Solution method that uses value propagation and digit elimination, but not explicit case-by-case analysis:
G1 = E9. (That's because of how they interact with G9.) That digit is not 2 because that would squeeze out digit 2 from the DEF123 square altogether. Therefore it is 8.