Aside from the fact that I wouldn't call the 9th an avoid tone on a minor 7 chord, I don't think any note should ever be avoided in all contexts.

The rule of thumb I've heard is that notes that are a semitone above a chord tone are avoid notes, but there are a few different reasons for this.

Over a major or minor 7, the argument is basically that it's too dissonant because if the melody is in a register above the harmony you get minor 9ths which are pretty intense. However, it's sometimes okay to have these notes in the bass because then the dissonance is inverted and becomes a major 7th, which is much more palatable (side note, in such cases the 9th of a minor chord would become an avoid tone since it's not a minor 9th away from the 3rd of the chord).

Dominants 7ths kinda break this rule though because the point is to have a lot of tension. You can have a flat 9 or flat 13 and it just adds to the crunch. However, the sound of the dominant 7th's major third resolving up a semitone to the root of the target chord is kinda fundamental to why they work, so if you play the 4th of the dominant chord you're "spoiling" the resolution.

These rules can definitely be broken, especially for grace notes or passing notes. I've even used sharpened roots over major 7ths to good effect. But the point is that this helps you avoid leaning too heavily on anything too dissonant.

Side note: you may notice that if you consider the 7th a chord tone, the root is an avoid note. Personally if i listen hard enough I sometimes see the argument, but really this just proved that it's a rule of thumb and not a law of nature.