In TeXbook p.156 it is said:
Suppose that you want to extend plain TEX by putting boldface math italic letters in family 9, analogous to the normal math italic letters in family 1. (Such fonts aren’t available in stripped down versions of TEX, but let’s assume that they exist.)
plain.tex contains this:
\font\preloaded=cmmib10 % bold math italic
Considering the fact that
plain.tex cannot be changed, I don't understand how (and why) this font may not be available in some versions of TeX.