People get shadowbanned here if they're obnoxious, or sometimes it's just bad luck. The comment you're complaining about was probably not specifically silenced, that poster is just in the hellban limbo.
If you like a comment (and maybe you need enough karma?) you can click through to it and 'vouch' for it, which makes it visible.
In the case of https://news.ycombinator.com/item?id=25398463 it was a combination of the trollish username (not allowed on HN: https://hn.algolia.com/?sort=byDate&dateRange=all&type=comme...) and the trollish early comments posted by the account.
We're always happy to unban accounts if their history shows that they are now using the site as intended. If you notice such a case, the thing that will actually help your fellow user is to let us know about it at [email protected] so we can take a look. You can also vouch for a [dead] comment that shouldn't be dead—the fact that banned accounts sometimes post good comments is the reason we created vouching in the first place.
(In the case of https://news.ycombinator.com/item?id=25398463, we could probably unban the account if they were willing to change the trollish username. I'll email to check if they want to do that.)
What's not so helpful is to post complainy comments about "mods deleting comments" and "echo chambers": they're not accurate, the fill the thread with offtopicness, and in the case where a user shouldn't be banned, they don't do anything to help that user.