That last one is great if it’s hidden behind a flag and some loud messages saying “failed to build, consider using —-python-only with performance limitation.”
Otherwise it’s going to be a source of mysterious performance issues with a single error message hidden somewhere in your CI or other logs.
I'm a bit mixed on a flag because C library dependencies are a fair support burden and an awful lot of people do not have workloads where a pure Python implementation is a significant performance issue because it's lost in the actual work done by the database.
I'd agree with something like a warning on first load or maybe something like an optional package so you could pin `psycopg3[cext]` if you want to guarantee the extension installed.
There is a mechanism to ensure that there is no unexpected regression. Exposing the PSYCOPG_IMPL env var a program can make sure to obtain a specific implementation and import fails if it's not available. https://www.psycopg.org/psycopg3/docs/api/pq.html
Otherwise it’s going to be a source of mysterious performance issues with a single error message hidden somewhere in your CI or other logs.