> I think this is a false dichotomy; in https://bugs.python.org/issue36274#msg351834 Jason proposed a few alternatives that allow for a secure and obvious default API while adding a new, explicitly unsafe API.

I'm not against that concept, but it is only appropriate for >= 3.9 as that'd be adding a feature.  This issue is marked a release blocker to decide what to do for 3.5-3.7 (and maybe 3.8 if deemed a serious breaking change).

> I'd like to add yet another option that may be useful specifically for maintenance releases: forbid only the problematic characters -- namely LF (and potentially CR and SP). This seems like a much more surgical fix for maintenance releases, allowing the null byte for CherryPy or the raw UTF-8 bytes for Swift, while still mitigating the CVE.

PRs with explicit tests for what is and isn't allowed welcome.  Thankfully for the UTF-8 case, its multi-byte codepoint bytes will never contain LF, CR or SP.