ASCII can be stored in 7 bit bytes on hardware configurations whose bus size is 7 bits, in contrast to the current consumer and server standard of 8 bits. In those contexts, it's compatible with UTF-7.
However, in practice, ASCII is stored on 8-bit bytes. Consequently, it's compatible with UTF-8.
ASCII compatibility is deliberate in both contexts. However, the compatibility refers to solely the first 128 characters. Any UTF code points greater than (7F)16 shall be incompatible.