**No**, that's very rude in my opinion. I think there are better alternatives.

###The answer needs additional details
If the original answer is correct as it is at the time of posting, but now requires additional info to cope with newer systems/updates, you can do one of the followings:

 1. Edit the answer yourself and add the details.
 2. Leave a comment with the details and suggest the answerer to append the details to the answer.

###The answer is completely obsolete (but was correct at the time of posting)
Just leave a comment saying it's out-of-date or just leave it as is. I believe users will see the date of posting/last edit and figure out that the answer might not work with a newer system.

If you have a up-to-date answer feel free to post it. If you think you have a (Ask Ubuntu) link to an up-to-date answer,  edit the outdated answer and append the link.

###The question itself is obsolete
You can vote/flag the question for closure. There is an option under *Off-Topic* which goes like this: 
>This describes a problem that can't be reproduced that seemingly went away on its own or was only relevant to a very specific period of time. It's off-topic as it's unlikely to help future readers.

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Of course if the answer is completely wrong/rubbish/potentially dangerous to begin with, downvoting (maybe also leave a comment pointing out the problem(s) with it) is the right action to take.