Available options:

1. Do nothing (keep cgi.log() and continue to use the default encoding for open()).
2. Remove cgi.log(). I think that the deprecation period is not needed because the function is not documented, is not imported by star-import, and is not shown in help.
3. Make cgi.log() using UTF-8 for open(). This may break some existing code if cgi.log() is ever used.
4. Completely rewrite cgi.log() using the logging module.

In all options except 2 cgi.log() needs to be documented and advertised as a new feature. And we should ask ourself: do we need this feature? Does it have advantages over the logging package?

It was more visible in 2.0. But since adding __all__ in 2.1 (in e99d5ea25ba994491c773d9b5872332334ccd1c5) it is a hidden feature.