Adafruit has made an I2C interface for four rotary encoders. It comes in the form of a PCB with four footprints that accept one of the most common rotary encoder forms (Bournes PEC11 horizontal style) ...
Anyone searching for a way to connect multiple rotary encoders on the I2C bus, may be interested in a new piece of hardware created by Simone based in Turin, Italy. The aptly named I2C Encoder V2 ...