Looks to me like it's in the "main" branch but not in 0.19.7. Maybe the release branch was forked before it went in? Maybe there's active development on a 0.20-targeted line, with some fixes getting backported to 0.19 as they're needed? I don't know. But I think it's in the development branch, but hasn't made it to production quite yet.

$ git checkout main
Switched to branch 'main'
Your branch is up to date with 'origin/main'.
$ grep 'Rewrite markdown' api_tests/src/post.spec.ts
test("Rewrite markdown links", async () => {
$ git checkout 0.19.7
HEAD is now at fc8280a9b Version 0.19.7
$ grep 'Rewrite markdown' api_tests/src/post.spec.ts
$