I'm never quite sure about the state of zlib. There are commits going in [0] but at the same time pull requests with fixes aren't seeing much traction (e.g. [1]).

Optimization work happens in forks like Intel's [2], Cloudflare's [3], and Chromium's [4], the latter of which also has a lot of added tests and fuzzers [5]. And there's zlib-ng [6] which tries to create a home for many of the improvements, but is at the same time yet another fork.

[0] https://github.com/madler/zlib/commits/develop

[1] https://github.com/madler/zlib/pull/525

[2] https://github.com/jtkukunas/zlib

[3] https://github.com/cloudflare/zlib

[4] https://source.chromium.org/chromium/chromium/src/+/main:thi...

[5] https://source.chromium.org/chromium/chromium/src/+/main:thi...

[6] https://github.com/zlib-ng/zlib-ng

(edit: line breaks)