What do you think about being able to hide a comment? Would that feature bring value to this community?
Would it bring value to us, probably.
Would it bring value to enough other Stacks to make it worth giving priority, maybe but probably not.
Would it happen, doubtful - the list of feature requests to the developers is quite long, and in all honesty there are dozens of things that by rights should come long before something like this.
If someone has said something spoilerific in a comment, and that information is either no longer correct/relevant or has since been incorporated into an answer, feel free to flag as "no longer needed".
If you're contemplating leaving a comment that would be revealing, feel free to use ROT13 or (say) ROT47 to conceal it (the latter has the advantage of handling numerics and symbols as well).