backtick-substitution
A deny rule: it refuses a backtick command substitution, which inside double quotes runs a command, and names what to run instead.
What it catches
A backtick command substitution, which inside double quotes runs a command.
Why
Inside double quotes a backtick starts a command substitution, so a pattern or a message carrying a literal backtick runs code: the backtick pairs with the next one anywhere on the line, and everything between them becomes one command. Measured here: a grep whose pattern and a later stage's pattern each held a triple backtick paired them into ONE substitution that swallowed the file operand and the rest of the pipeline. What was left was a grep with no file, reading a stdin that never closed, and it held a subagent for two hours. A literal backtick belongs in single quotes, where it is text. A substitution is written $(...), which nests and cannot pair with a stray backtick. Backticks inside single quotes and inside a quoted heredoc (<<'EOF') are text and never fire; neither does a line that does not parse.
Seeing it
A verdict names its rule in brackets, which is how you got here:
deny [backtick-substitution]: ...
magus describe rule backtick-substitution prints the same entry at a terminal, and
magus describe rules lists every rule this workspace enforces.