Learn English – Is untasty a valid English word

Because people use "untasty", it is a word. It also has a long history, since it dates back to at least 1566 (assuming it only entered the language once):

If, one..drincke nothing but vinaiger, Vntastie and vnfyne.
_{A medicinable morall}

It is listed in the Oxford English Dictionary, with the above quote and another from 1733 but the entry hasn't been updated.

It's easy to understand its meaning because it's formed by derivation of an existing word. Given that it's not in many dictionaries, it's probably not very popular. In any case there are plenty of widely-used alternatives: yucky, gross, disgusting, etc. However, that isn't to say it's not used at all. A quick search brings up many results of it being used recently in publications:

However, there are some limitations to this intuition, depending on the extent to which healthy food actually is considered untasty [...]
_{Affective Determinants of Health Behavior}

No one wants to eat a food that is untasty or spoiled. Jesus is the Bread of Life. He is the spiritual bread; and no bacteria can destroy it. For the spiritual bread is everlasting, and can never be untasty or spoiled [...]
_{Main highway to God's kingdom}

Three or four untasty-looking pumpkins came falling down with the vines.
_{Twenty-four Eyes}

After sampling both, my own judgment is that pitcher-plant water is generally less untasty than bog water, but that the likelihood of protozoan presence in the pitchers would give preference, for safety's sake (should the need arise), to relatively plankton-populated bog waters.
_{Wildflowers of the Eastern United States: An Introduction to Common Species}

For many years, he has fed his pigs the food left over from the local university cafeteria, which is known to be low in protein, deficient in vitamins, and downright untasty.
_{Loose-leaf Version for Genetics Essentials: Concepts and Connections}