We currently hide all the warnings that the dependent crates have. If you cargo build -vvv you can see them all in glorious technicolour.
One way to nudge people into thinking more about their dependencies might be to output a count as standard of how many compiler warnings there are in your dependencies.
pinned dependencies will get older and as you upgrade your rustc version (which people invariably do do), that count of warnings would increase reminding them that maybe they should cargo update.
To me this all feels win win and we would trend to an ecosystem with less warnings in over time.