If the erased letters series were b,c,…h,…m and some others, my criterion would be “leave only the letters that by adding each of them before word AT no valid English word will be created ( bat, cat, hat, et al).

Using another word instead of AT, exactly 15 words of 2 or more letters may be created. Find this word. 

All newly created words must be approved by Google. List them, now it is a D2 puzzle.

2. Bonus 

Find another word of any length, which, by the same reasoning, creates more than 15 words.