I always used to do this but you can train yourself out of it. I didn't realise I never looked up until at some point my ballet teacher started commenting on it (I am a dancer). She focussed on it with me for a while, mentioning it in nearly every class and I realised I did it in day to day life but it is less obvious. When dancing, heads are so important so I decided to work on it. To try and make it natural/a habit I decided I had to do it in outside life as well. To start with it was really difficult: not only would I constantly forget and find myself looking at the floor but it felt very dangerous in a few different ways. Maybe because I'm short sighted (though this is corrected with lenses) I tend look just in front of me to make sure I'm not going to trip on anything so it felt a bit physically dangerous, like walking with your eyes shut, until I got used to it. But it also felt dangerous because there was the chance of meeting someone's eyes (hadn't even realised I was avoiding them - this was years before I got diagnosed) and having to know how/whether to acknowledge them. And it also felt incredibly arrogant or something. I am very shy by nature though have got better, and used to basically just try not to be noticed if at all possible, so to walk with my head held straight felt similar to if I had been contributing to conversations or giving opinions which at that point in my life would have been unthinkable.
But I worked on it, and it does now come naturally. I still worry about eye contact and acknowledging people though not as much, but I love to look at the sky and trees and things around me which I didn't notice as much before. I think it helped me to have a concrete reason to do it, somebody to remind me every day or two and an explicit way in which I was supposed to use the head and gaze.