It seems miniz_oxide backend has matured enough to be the default:

1. Now that miniz_oxide backend contains no unsafe code and calls to it no longer go through a C API, it is strictly safer than the default miniz backend.
2. Performance is about the same (oxide decompression is faster, compression is slower).
3. miniz_oxide has received plenty of testing since the popular reqwest library uses it exclusively.

Is there anything else needed before flate2 can switch to miniz_oxide by default?