I used I2S to connect my Metrum Acoustics streamer with their DACs and found it interesting that the designer uses an RJ45-type connector rather than the HDMI connector/cable used by Denafrips and most others. The lack of an I2S standard is a little frustrating but maybe a result of that connection method being used for short internal connections within one component, as discussed by @soix, and not for external connections between two components.
I have yet to hear a meaningfully discernable difference between properly constructed digital cables, and would suggest getting something well-built, like the recommended DH Labs cable, but spending the big money elsewhere in the system.