Backend broken in plain ptex and uptex

[blefloch]

Running pdftex expl3-generic (or more generally inputting expl3-generic.tex in a larger file) works fine and loads l3backend-pdftex.def. Same successful result for xetex or luatex. But eptex expl3-generic gives

This is e-pTeX, Version 3.141592653-p3.9.0-210218-2.6 (utf8.euc) (TeX Live 2021) (preloaded format=eptex)
 restricted \write18 enabled.
entering extended mode
(/home/bruno/texmf/tex/latex/l3kernel/expl3-generic.tex
(/home/bruno/texmf/tex/latex/l3kernel/expl3-code.tex
(/home/bruno/texmf/tex/latex/l3kernel/l3deprecation.def))
(/home/bruno/texmf/tex/latex/l3backend/l3backend-dvips.def
! Undefined control sequence.
<argument> \g__kernel_backend_header_bool 
                                          
l.65 \bool_if:NT \g__kernel_backend_header_bool
                                               
? 

[josephwright]

I'll sort this