Feeding
\documentclass{article}\usepackage[T1]{fontenc}\usepackage{newtxtext}\usepackage{microtype}\begin{document}\textsc{}% doesn't matter what text we enter here\end{document}to pdflatex prints
Package microtype Info: Character `029' is missing(microtype) in font `T1/ntxtlf/m/sc/10'.(microtype) Ignoring protrusion settings for this character.to the log (sometimes we do search for missing in the log to check whether there are any problems). If we drop \usepackage[T1]{fontenc}, we get a complaint about 013 instead of 029.
Which character (029 and 013 are just numbers and not characters) is missing where? What does the aforementioned message in the log tell us, especially given the fact that no text is output via \textsc? (You can even suppress the page number with \pagestyle{empty} to get really no text on the page at all.) Is this message something to worry about? If so, who should worry, and, (most importantly 😁) who is the culprit?
EDIT (based on the comments and answers of Ulrike Fischer, rallg, and Stephen): The message in the log is useless for the very input above and even annoying because it takes up space and distracts the author without need. However, if the input contains \textsc{fl}, we'd better be informed or even warned that the output does not contain a real ligature (because the output has the small capitals ꜰʟ instead). We still need the (now present) additional warning “Missing character: There is no ^^] in font ntx-Regular-tlf-sc-t1!” message in the log if the input contains \textsc{\char29}, and we wish that the part “^^]” be a bit less enigmatic.
