I try to use `expl3` in plain TeX with the code:

``````\input expl3-generic %
Some text
\bye
``````

I have en error message with all engines. For example, with `pdftex`:

``````! I can't find file `l3backendpdfmode.def'
``````

I suspect an hyphen to be lacking in `expl3-generic.tex` (date 2019-07-01), line 89.

• There is a hyphen missing in the file. It should look for l3backend-pdfmode.def. – Ulrike Fischer Jul 25 at 14:17

Yes, there is a missing `-` here. We have a fix in-hand in the development version: this is likely reasonably urgent, and I will arrange a CTAN release later today.