Several bugs were fixed and the new backport available is 0.23 (0.23.0-1~bpo9+1) So I went ahead I installed the package but 0.21 (0.21.1-1~bpo9+1) was installed instead.
Something I notice in the instructions was this message: “Certbot’s DNS plugins which can be used to automate obtaining a wildcard certificate from Let’s Encrypt’s ACMEv2 server are not available for your OS yet”
The tracker is packager-focused and can be really confusing to end users. If you just want to know what the latest package you can install with apt is, use packages.debian.org, where the entry for certbot correctly indicates the binary packages in stretch-backports are still at 0.21.