Find Abecedarian Words
Jelly, 9 bytes
fØẠŒlÞ$ƑƇ

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-3 bytes thanks to Nick Kennedy
fØẠŒlÞ$ƑƇ  Main Link
        Ƈ  Keep elements that are
       Ƒ   Invariant when
.[][.]$    Last two links combined, but [ØẠ][ŒlÞ] is an LCC, so it takes another
f          Filtered to keep
 ØẠ        Alphabetical characters
     Þ     And sorted by
   Œl      Lowercase

Basically, &quot;keep elements that are invariant when you filter out non-alphabetical characters and sort alphabetically&quot;. Jelly's sort is stable.