disowned.

Кстати, здесь отличная двусмысленность :) которую фиксирую.

The English is good. The speaker is chiding someone for failing to tell.

I think the meaning should be something like:
Perhaps you have told me about it long time ago (but I can't remember it now).

Ambiguos.

That's true, it's possible to read this English as expressing uncertainty about what may or may not have been said too long ago to remember.

Are the other linked examples ambiguous in exactly the same way? Including the Russian sentence we started with?

If not, I'll give the English a more definite sense. But I could use a little advice on how an issue like this is resolved. It must arise over and over again.

The Russian translations each reflect one of the two possible meanings. In fact, the other translation was added by me to demonstrate ambiguity.
I believe, and I believe it is generally believed so, that occasional ambiguous examples are good for the learner, demonstrating them where and how confusion could arise without proper context and also reminding them of polysemy that many words and expressions exhibit.
I believe in being generally descriptive and applying as few limitations to what one may add as possible.

Would the best approach be three linked examples in English?

1) Ambiguous
2) Resolved in favor of the first meaning
3) Resolved in favor of the second meaning

I take the point about polysemy, but is this reflected in the most useful way?

You can link English to English as well if you deem fit, I believe, just that it be a rather rare occasion.
This is a rather rare occasion - you don't bump into ambiguous phrasings every other hour, right? - and your reasoning makes sense to me, and maybe it would make sense to others as well but we can always wait for them to voice their take on the matter.

I'd love to see a policy statement about multiple meanings from the project administration.

Is this discussed somewhere I haven't been yet?