Yes. You can enable "showdead" in your profile to see [dead] comments hidden by default. Comments end up [dead] when they have been [flagged] by enough other us

Yes. You can enable "showdead" in your profile to see [dead] comments hidden by default. Comments end up [dead] when they have been [flagged] by enough other users, when the user has been explicitly [banned] by the mods or if the user tripped an automated system (bot detection, sockpuppet of a banned account, lots of [flagged] comments...) If you think a [dead] comment would contribute positively to the conversation, you can vouch for it, in which case it will become visible to everyone once enough people have vouched for it.