# If you'd like to omit non-matching lines from the result; add ';d' to the end of the expression.
sed -E 's/\s*\{\{source\|(?>[^{}]|(\{\{(?:[^{}]++|(?1))*}}))*}}\s*//gm;t' <<< "# Test is a cool word {{source|{{nom w pc|Chantal|Bouchard}}, ''La langue et le nombril'', Presses de l'Université de Montréal (PUM), 2020, p. 174}}.
# Test is a cool word {{source|Nicolas|Daniel, Presses de l'Université de Montréal 4}}"
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for SED, please visit: https://www.gnu.org/software/sed/manual/html_node/The-_0022s_0022-Command.html