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An update to the article. On my system, not only was Droid Sans Mono no longer present due to being removed from the package repositories, the default monospace fallback was not working properly. I verified this by running fc-match monospace, which returned DejaVuSans.ttf: "DejaVu Sans" "Book". I'm not sure what caused this, but I was able to fix it by creating a file /etc/fonts/local.conf with the following contents:

<match target="pattern">
    <test name="family" qual="any">
        <string>monospace</string>
    </test>
    <edit binding="strong" mode="prepend" name="family">
        <string>DejaVu Sans Mono</string>
    </edit>
</match>

After that I ran sudo fc-cache -f to rebuild the font cache. The monospace family was then correctly set, fc-match monospace returned DejaVuSansMono.ttf: "DejaVu Sans Mono" "Book".

This might have been a problem specific to my system, as I couldn't find any existing bug reports describing the same issue, although there were a number of similar ones. Most commonly affected programs seemed to VSCode and Firefox, although in this case it was an underlying system issue. This Stack Exchange post was helpful in working round the problem.