The emacs community might think it's "working fine". The glibc community-- which had to forego many bugfixes and optimizations for years, solely because of emacs-- might disagree.
unexec is hairy, but it's mature, greybeard hair. It is no worse than any JIT, for starters.
But now they want to deprecate malloc_{g,s}et_state(), without a plan to improve ptmalloc2? They already failed.
Deprecating an API for no good reason is failure, not an improvement. It not only breaks emacs, it breaks other software also. unexec is used in perl5 also, btw. just not in the official perl5 packages.