Many popular NodeJS packages are moving towards ESM-only support. This leads to the necessity to build applications with ESM support as well.
Some dependencies may be built for both, ESM and CommonJS. If, however, deeply in the dependency tree there is an ESM-only package, the application itself must be ESM. It does not suffice to make sure that your own packages are built for ESM if you choose to stay on CommonJS.
NestJS itself currently does not officially support ESM and there are also no plans to change it.