Is there a lack of availability of OsF (old-style figure) faces available these days? 99% done on a book layout job, entirely in Janson Text from Linotype, but now must find and purchase the italic OsF, which, surprisingly, Linotype no longer seems to offer. Has OsF fallen so out of favour that it's rarely offered any longer, and why?

The Family package from fonts.com doesn't have OsF:
https://www.fonts.com/font/linotype/janson-text

3 rather dubious and crap-looking sites appear to offer it, but look totally unsecure. Why isn't this font available from somewhere reputable?

Strange. Any rate, many thanks in advance...

Putting things like “OsF” in the font title, was a practice before OpenType, when the character set of (PostScript Type1) fonts was very limited. Today, the choices are usually added to a single file. Linotype’s Janson Text has various figure sets available, including old-style figures:

download at MyFonts

 

Ralf, thank you for the clarification. Much appreciated.