What links 617 Squadron with mistress of a Bourbon ?

I see that the motto of the RAF No. 617 Squadron is "Apres moi, le deluge" (there should be accents on the first and fourth words). This has been attributed to both Bourbon King Louis XV and to Madame de Pompadour. In some quotations, it is written as "Apres nous, le deluge."

Interesting motto for an RAF squadron.

https://en.wikipedia.org/wiki/No._617_Squadron_RAF

I've always assumed the motto was adopted after, not before the "dambusting" raids. See also the flood waters pouring through the dam on the squadron's badge shown on that link.

Both the motto and badge were adopted by 'the Dambusters' after the raid. There wouldn't have been much point in adopting it beforehand as that would have destroyed the curtain of secrecy under which the raid was planned and executed. That the motto was a quotation from centuries before is irrelevant.
According to Paul Brickhill in his notable book "The Dambusters" the squadron badge of 617 Squadron was chosen after a competition within the squadron to design it. The badge was chosen by King George VI on a visit to the squadron after the raid, with the motto 'Apres moi, le deluge'. However, on submission to the College of Heralds for approval the motto was questioned, as it had been used by Marie Antoinette in an 'unsuitable' context. When resubmitted as "Apres nous, le deluge" that too was questioned as an an aged Greek had used it to show selfishness. Only after it was pointed out that the King himelf had chosen both badge and motto was the emblem given approval as the Heralds had 'no wish to interfere with the Royal Prerogative.'