It's the norm in the UK and we successfully forced this in Poland (though posters almost NEVER post salaries here). How? The companies need IT staff so much that almost all IT job boards (at least the most popular ones - like FB groups or https://nofluffjobs.com) started requiring the salary range.
I think your idea is praiseworthy, but I'd never ever create a website like this with hidden salaries. Especially in your case - it's so cool people post jobs on your board, but what if they do so, because they're offering 10, 20, 40% less because it's a place for "old geeks that noone wants"?
I'm really super proud that if a IT ad in Poland has no salary range most of us just ignore it. And it took us maybe 2 years to get to this place. I think every other country should follow the lead and end the "competitive salary" trend. I don't want to spend 3 days on interviews just to discover that the salary offered is way too low for me. Salary missing from an ad is a big lack of respect, the sooner people realize that the better.