Any of the 3.1.x releases are not considered as main 3.x releases and so as they get surpassed by the next minor point release, they are considered deprecated. At least that is my understanding. 3.1.4, 3.1.7 and 3.1.9 are considered stable so those stick around. The goal is for all 3.1.x releases to be backwards compatible so if you are on 3.1.5 now, upgrading to 3.1.7 or 3.1.9 should be fine. breaking changes are only potentially introduced in changes between 3.x releases or soon to be 4.0.
If you just point your ood server to:
[ondemand-web]
name=Open OnDemand Web Repo
baseurl=https://your.local.repo.com/ondemand/3.1/web/el9/$basearch/
enabled=1
gpgcheck=1
repo_gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-ondemand
[ondemand-web-source]
name=Open OnDemand Web Repo
baseurl=https://your.local.repo.com/ondemand/3.1/web/el9/SRPMS/
enabled=0
gpgcheck=1
repo_gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-ondemand
You should then install ood with just yum install/update ondemand and it will pickup what ever are the newest releases, which in this case should include ondemand-3.1.9-1.el9.x86_64.rpm which is the actual main ondemand payload.