This is still possible if you're a Strava subscriber. If you're not a subscriber, however, then you can't see full leaderboards, irrespective of whether people allow their rides to be visible. You can see the top ten but you can't break things down into age groups, time periods, age, etc. meaning you can't do what you're trying to do.
As a separate point there are two settings under Settings > Privacy Controls which affect whether rides are visible and where.
- Activities: if you don't set that to 'Everyone' then nothing appears on leaderboards for segments.
- Flyby: this only affects whether you are visible on Flyby.
I believe the default on Activities is 'Everyone' still (but I'm not certain). The default for 'Flyby' is definitely 'No one', however. They changed that a while ago (for what are probably spurious, legal concerns). Gradually, people are noticing and opting back in, but it's taking a while.
Anyway, the main thing preventing your usage is almost certainly the need for a subscription to allow full viewing of leaderboards.