Dear Maglog,
ESI is applicable to all factories in Maharashtra. Since 1978, it has been applicable to shops, hotels, cinemas, restaurants, and newspapers employing 20 or more employees all over Maharashtra (as per recent amendment). Therefore, since IT companies sell their products to clients and customers, they will be covered within the ambit of 'shop' as per the ESI Act. Hence, it is applicable if they are engaging more than 20 employees.
Thank you.