Here is an app note from Atmel describing how their TWI (I2C) peripheral, found in tinyAVRs, avoids SCL clock stretching.
This application note describes a TWI slave driver for the ATtiny20 and ATtiny40. Aslave software driver is included. The application note explains how the TWI SCL clock is not stretched, provided that the AVR clock speed is fast enough to
release the serial clock (SCL) and the acknowledge (ACK) on the serial data (SDA) signals.