What is hardened SHA-1, how does it work and how much protection does it offer?
Hardened SHA-1 detects collisions built of a certain form, If someone were to find a collision using brute-force birthday attack (currently not feasible) the detection would not work. 

The vectors are specific small differences which may help to convert a near collision into a full collision. 

The details are in the paper: https://marc-stevens.nl/research/papers/C13-S.pdf 

This will detect an attack which has a similar construction to the attack performed. Future cryptanalysis may however lead to other forms of attack. It is worth mentioning  any hash function that is may be vulnerable to new kinds of attacks though. SHA-3 may also be vulnerable to attacks that are yet unknown.

Hardened SHA-1 tries to be identical to SHA-1 almost all the time with a probability of differing in honest usage of less than $2^{-90}$. Yet it should be resilient to attacks by detecting them and changing the output to something that differs.

SHA-1 has been collision free until very recently. Hardened SHA-1 should be secure in this regard; much more secure then SHA-1. It is obviously better than thoroughly broken options such as MD5. 



Hardened SHA-1 tries to be backwards compatible with SHA-1. If you don't need this backward compatibility don't use it, use SHA-3 or SHA-2, even if you have to truncate to 160 bits.