What font is used in Lightroom icon?
This is most likely Adobe's internal corporate font known as Adobe Clean. It is not available for retail purchase. The closest commercially available font design would probably be FF Dax (FF Dax® - Webfont & Desktop font « MyFonts))
I know that Adobe Clean is not available to buy, but Clean letter r is different.
It's entirely possible they customized it - they love to do stuff like that.