You are absolutely right, Rob. I should have said that the practice of using slurs for all grace notes as a matter of course as at present started in the middle of the 19th century. Before then, it was hit or miss and actually rare for many composers. The fact that these slurs were so often omitted shows that were clearly felt to be unnecessary not only by many composers, but also by publishers. In my opinion, they were generally right to omit them because they can clutter up the score for no good reason.
The first edition of the last movement of Mozart’s Sonata K. 457 contains 22 appoggiaturas written as small notes. None have a slur. A quick look through the manuscripts of Mozart’s other piano sonatas shows much the same. He rarely (if ever?) slurs single grace notes, but sometimes will slur larger groups such as slides or turns. The New Mozart Edition adds the “missing” slurs.
Of the many grace notes in the first editions of 32 Beethoven’s Piano Sonatas, the vast majority are not slurred. I found only 4 slurred pairs. Beethoven most often doesn’t slur larger groups like slides, turns or arpeggios written as small notes. A look through a bit of his other music showed the same.
The manuscript of the middle part of Chopin’s Mazurka op. 6 no. 1 contains 56 grace notes. None are slurred. Other works like the Preludes and Etudes do not use slurs for grace notes. For example, the “wrong note” Etude op. 25 no. 5 uses no slurs for its many dissonant grace notes. However, Chopin generally slurred small notes when they were an alternate way of showing a broken chord.
As late as Brahms’ Handel Variations (1861) the composer is not slurring grace and other small groups of notes in his manuscript, which was engraved somewhat faithfully in the first edition and the later Breitkopf Complete Works.
Scarlatti’s copyists slur grace notes far more consistently than Mozart, Beethoven, and Chopin. But Scarlatti was working in isolation from the mainstream. J. S. Bach sometimes slurred small notes and sometimes didn’t; it is possible that this was intentional and indicated the rhythmic position and speed of these notes.