On SE, contribution licensing was a sticking point, especially when the sites were relicensed.
Might I suggest that each site have a list of acceptable content licenses, both current and former. Each post made should be made under one of the then-current licenses (as selected by the user if more than one is current) and that licence referenced in the contribution record.
Then, if the site changes its licenses(s), any pre-existing contribution remains under its original license unless the licensor decides to update its license.
This has the effect that each contribution is explicitly licensed under a known set of license T&Cs, and that relicensing cannot change existing licence conditions.
If, in our future website, we ever come across a need to change the license, we can: (after discussing with the community, of course):
Update our footnote to say something: āAll content licensed under [license here], except where otherwise noted.ā
Immediatelly implement a small subscript under every question and answer stating the content is licensed under [previous license].
Give regular users a straightforward way to agree to re-license all submitted content as [new-license] (or, additionally, re-license contributions on an individual basis, for those who might want this for whatever reason).
As users approve their content being re-licensed, the footprint note under each post is no longer shown.
Given that itās unlikely SE is going to resolve their own licensing issue, if any sites are going to import posts from SE then this should really be part of MVP, so that posts from before the CoC change can be marked as CC BY-SA 3.0 and posts after can be CC BY-SA 4.0 (and ideally the really early ones can be 2.5).
This way, people can use content with a different license if they have a particular reason to, but everything has a reasonable default with explicit permission to additionally license content submitted under that default with a later default as well.
TopAnswers does this well: as a user you choose a default license for all your posts, which you can change for an individual post at creation time. This approach also plays well with the need we (and others) have already identified that sometimes content within a post needs to be licensed differently (code, work for review, etc).
Initial imports from SE should be date-based because thatās the only info we have. But we donāt need to repeat the license-ambiguity problem on our site. Store it explicitly, and either display it with each post (could be noisy) or display only when different from the documented site default.
TopAnswers are still mulling over what to do with imported content because SEās situation really is a mess when it comes to content initially posted under one license and then edited under another. We will probably link all imported content to a post explaining the ambiguity, and the link will probably look something like: CC BY-SA 3/4?
One thing that some SE sites have is a āciteā button. The license could be put in there (or a similar button), or perhaps on the āquestion discussionā page or some such.
Point is, we donāt necessarily need individual license details immediately visible in each post (although, sure, some communities might want that), so we can put it somewhere easily found but not in the way.