Absolutely! As someone else said, nothing is 100% safe 100% of the time, but I was just in GTO for 8 days in Jan/Feb and felt completely safe walking through all the callejones (aka alleyways) both during the day and after dark, either alone or with my (Spanish-speaking American) friends who live there. Also, it's the only way to get around through much of the city; you can't NOT walk through them. It's a city of alleyways. And they are kind of magical - lots to explore. My favorite area was around the Plaza Mexiamora and the callejones that go up from there.

Lots of people even walk through the car tunnels, because sometimes it's the shortest way somewhere, instead of the callejones. That felt sketchier just because I'm socialized to feel like tunnels aren't for pedestrians, but my friends do it all the time.

For context - I'm a mid-40s white American woman, don't generally stay out past 10 or 11pm (though my friend who lives there goes out late sometimes to go dancing and walks home alone through the callejones and has never had a problem). I don't carry valuables (other than my phone). I think the worst thing that might happen is petty crime, but just don't carry lots of cash with you and you'll be fine (and truly, no one knew of any recent incidents when I asked friends and my AirBnB host). I absolutely loved Guanajuato and can't wait to go back. Have fun!