Quite simply, none of the available reactions has a clear and pressing need for such a marking.

"Works for me" can be directly contradicted

The only good reason to mark a "works for me" reaction as "stale" is because the solution is now obsolete - and there is already a reaction explicitly for indicating that. We don't have to predict a timeline for a hypothetical Python 4; if and when it happens, we can mark answers that are 3.x-specific after determining that they are 3.x-specific (while leaving alone answers that aren't affected by the version bump, but are just as old).

"outdated" and "dangerous" tags don't go bad

As already noted.

If a technique is outdated, it can't stop being outdated.

If a technique is dangerous, it is extremely unlikely to become safe automatically; even if new library support appears, the answer will probably have to be edited to take advantage. It would be incredibly poor practice on the part of e.g. a library maintainer to secure a fundamentally insecure technique, years later, and reuse all the same names for it.

Even if it did happen that version X of a library makes previously-unsafe technique Y safe, with no required alteration to the code, the answer is still in a sense outdated - because it doesn't account for the library version required for security (a version number that was unknowable at the time of writing).

If an answer is edited, years later, such that it goes from being outdated or dangerous to not being thus, then that in turn can be disputed by adding appropriate "Works for me" tags.

"interest" becoming stale isn't relevant

Either the site went live, in which case there's no reason to care about book-keeping on the process that got that far; or the proposal has been in discussion for so long that it's probably time to face reality and reject it (probably without prejudice); or else it's one specific user who has reconsidered the proposal and should be responsible for actively retracting the reaction.

Because answers do get edited in ways that could immediately and directly invalidate a reaction, and because people do leave communities over time (and therefore could leave a reaction and then fail to see an edit that invalidates the reaction), and because sometimes people will just stubbornly disagree that their reaction is invalidated: I agree that we should at least be able to see the dates for reactions. Expecting a mouseover might be too much; I like OP's idea of reduced saturation for older reactions - but instead applied to reactions where all of the following apply:

• the question has been edited;
• the reaction was made before the most recent edit;
• the most recent edit is older than some time threshold (either configurable by community, or dependent on the community's level of activity);
• the reacting user has been relatively inactive since that edit.

Mouseover guidance would suggest something like: "it's possible that the reacting user hasn't fully considered edits made after making the reaction".