I have been told by @fyrepenguin, in this discussion, that « edits are not supposed to invalidate existing answers ».
Does this rule exist; if yes, where is it explicited? practically how is it enforced?
Yes, from How do I use the Suggested edits queue?:
Common reasons to Reject
- changes a post to say the opposite, or something very different from what the original post read.
It's certainly a well-recognized principle, based on common sense1, and necessary to keep the site working properly. I am sure there are several posts in the main meta where consensus can be established.
But if you are looking for an "official list of rules" that enforces common sense and consensus you may not find such a list.
practically how is it enforced?
You just saw it! For edits by users below a certain reputation level they go into a review queue first and are voted on. For those above, it's assumed that common sense and understanding of how SE works has taken root.
And if not, then other community members that have common sense and a respect for concensus may notice such destructive edits and remind the editor not to make them.
In a nutshell, Stack Exchange is about generating good answers to on-topic questions.
If the question becomes a moving target; if the question morphs to a somewhat different question after one or more user writes an answer, then several bad things can happen, including but not limited to:
...unless the existing answer authors concur with the change.