[lucb1e]

> An obsolete flag would need to be prominent

The way I (not the person you replied to) imagine this is that the post goes to the bottom and gets a red background color or is faded out, similar to how deleted answers are shown in red (if you have enough reputation; in this case everyone should be able to see these) and downvoted posts are faded out.

Edit: this is the background deleted posts currently get, for those who aren't active on the SE community: https://i.stack.imgur.com/EDAIF.png

[not2b]

But movement of a post to the bottom doesn't help users who find it directly via a Google search. If it was a high-quality answer in 2017 and is badly wrong now, it may still be a highly ranked URL in Google's database. So the obsolete flag would need to prominent for people who follow a direct link the answer.

[lucb1e]

Did you also see this part?

> and gets a red background color or is faded out

And most of the time, a newer answer is available (at least in my experience) so for those the downranking would also help.