I have been using \includegraphics with plain TeX without problems until yesterday, when I updated TeXLive and wrote (minimal)
\input miniltx
\input graphicx.sty
\includegraphics{espec.png}
\bye
The compilation showed
! Undefined control sequence.
\Ginclude@graphics ...\Ginput@path \set@curr@file
{#1}\edef \uq@curr@file {\...
\Gin@ii ...eys {Gin}{#1}\Gin@esetsize \the \toks@
\endgroup \fi
l.6 \includegraphics{espec.png}
and stopped.
graphicsrelease, and should cover plain – Joseph Wright Oct 10 '19 at 16:47