I read somewhere that you can only travel 90/180 days from a few years back, but I wonder if that's still the case, and if it's enforced given open border.
Yes, you can leave Germany for as long as you want with a youth mobility visa. However, you cannot spend more than 90 days in any 180-day period in other Schengen countries with that visa. You could exceed that limit in another Schengen country if you had another D visa from that country. Similarly, if you go to a non-Schengen country such as Canada or the UK, you can stay there for as long as that country will allow you.