My initial testing of the ATtiny85 running at 1mhz as an i2c slave with the ESP8266 as the master was a failure, just could not talk. So I tried running the ATtiny85 at 4mhz, this was better as it worked most of the time. So I did some investigation as to why 4mhz was not consistent and found that the root cause is the default limit for i2c clock stretch configured on the ESP8266 was too short for a slow ATtiny85. The default is 230us. So I increased it to 1500us and tried the ATtiny85 at 1mhz again. Success!!!!