Does the Appendix Probi specifically talk about what correct "pronunciation" should be (of the day), or correct "spelling"? Does "Idem non ide" mean that you shouldn't say "ide" or write it? An English document about spelling (in the UK) might read "night not nite", which one could read as saying that English people say [naicht], but no one in England has pronounced the 'gh' in 'night' for about five hundred years and the spelling system represents an archaic pronunciation that has been conserved as the official standard. I haven't read anything other than the list of 'mistakes' in the Appendix Probi so would be interested if anyone has any more details. Thanks.
The Appendix Probi reveals errors of both types, i.e. of orthography and pronunciation. The sources for the content of the Appendix are written (as Barnett notes below), but, as such, they must be understood as reflecting habits of pronunciation.
In the article “The ‘Appendix Probi’ as a Compendium of Popular Latin: Description and Bibliography”, the various types of errors were categorized as follows:
A great variety of linguistic features and developments are, indeed, attested in the condemned forms, which follow the word "non" in the complete text given below. Five of the most obvious word alterations, together with one example of each from the Appendix, are:
- Case errors in compound words, as in nobiscum non noscum (#220)
- Other problems in compounding, e.g., aquaeductus non aquiductus (#22)
- Change of declension in nouns, e.g., palumbes non palumbus (#99), and in adjectives, e.g., tristis non tristus (#56)
- Change of noun ending and declension to agree with usual gender endings, e.g., socrus non socra for "mother-in-law" (#170)
- Use of diminutives, e.g., auris non oricla (#83)
This list also contains many items that attest to phonological features of everyday Latin:
- Syncope, frequently after the stressed syllable, e.g., speculum non speclum (#3)
- Assimilation, as in umbilicus non imbilicus (#58), and dissimilation, as in terebra non telebra (#125)
- Gemination, or doubling, of consonants, e.g., drac non dracco (#110)
- Reduction of -ct- to -t-, as in auctor non autor (#154), of -ns- to -s-, as in mensa non mesa (#152), and of -rs- to -ss-, as in persica non pessica (#149)
- Loss of h, adhuc non aduc (#225), or of final m, eg. numquam non numqua (#219)
Therefore, the particular error you describe, i.e. idem non ide, would be categorized as a phonological error (the second list, item #5), namely, the loss of final m.
Having said that, the evidence suggests that the Appendix primarily deals with questions of orthography as opposed to pronunciation. In his article, “The Sources of the ‘Appendix Probi’: A New Approach”, F. J. Barnett says the following:
But perhaps the most fundamental point on which the assessment of the Appendix is affected by this new identification of sources concerns its status, whether as an attestation of spoken or of written vulgarisms. At one time the former was probably the prevailing view. But the sources here identified are exclusively written, not perhaps absolutely excluding oral influences — after all a person who reads may also listen — but severely limiting their scope. It is no longer possible to see the Appendix as a vessel replenished from the 'gushing spring of the living language', as Bachrens styled it, or as 'notes on current errors of speech', as L.R. Palmer, more soberly put it (The Latin Language [London 1954], 154). That does not, of course, mean that the text has lost its value as evidence of changes in pronunciation, since vulgar spellings often reflect vulgar pronunciations.
Barnett, F. J. “The Sources of the ‘Appendix Probi’: A New Approach.” The Classical Quarterly, vol. 57, no. 2, 2007, p. 736. JSTOR, www.jstor.org/stable/27564103. Accessed 7 May 2020.
Quirk, Ronald J. “The ‘Appendix Probi’ as a Compendium of Popular Latin: Description and Bibliography.” The Classical World, vol. 98, no. 4, 2005, pp. 397–409. JSTOR, www.jstor.org/stable/4352974. Accessed 7 May 2020.