Whilst it is 95% calories in calories out, keto (not low carb, as low carb doesn't include high fat) can be good for muscle retention whilst in a defecit - as more foods that you consume naturally have higher protein (I utilise keto when looking to drop body fat, consuming a lot of slightly higher fat cuts of meat as a replacement for the carb calories, so chicken thighs instead of breast, 10% ground beef,etc). The higher fat content correlates to higher testosterone count, and higher protein means greater muscle retention.