Now since Oct 2018 no new tag release was created on  however the CI (same which build the OCP 3.11 managed by RH) evolved and switched to a different method where it no longer cut "OKD 3.11 tag releases" but it started to create artifacts on a rolling mode - e.g when new commits were/ are being merged into 3.11 OKD branch .
With that said the OKD 3.11 is formed by Docker images + the rpms generated by RH CI (same used by OCP 3.11) which are available at 
As you can see from above PaaS SIG was filling a gap with creating the rpms for OKD 3.x until CI catched up and started to build the OKD 3.11 rpms
With that said we - PaaS SIG - have no longer released newer 3.11 OKD rpms into CentOS repo after the switch over mainly because
- the community or folks asking for newer OKD 3.11 rpms in CentOS went all quiet, in other words nobody was asking for anything new
- the confusion on the OKD 3.x vs OKD 4.x and who does what etc => that was clarified with the formation of OKD Working group where things became more clear
Regarding your question around "how frequently the PaaS SIG is going to release and from which OKD branches" my answer is
Currently i haven't planned for any newer OKD 3.11 rpms to be built from  branch (not taking into account the release tag but the latest commit in the branch) and published into CentOS repos however IF there is a demand i'm happy to resurect and change the automation to get newer rpms built.
However i want to make it clear that if if the above work will be done, it will be as long as RH will keep supporting 3.11, when OKD 4.x will go GA then i think OKD 3.11 will no longer get updates.
I hope my long answer does clear any confusion.
Regarding "when OKD 4.x will be release, the cadence etc", as you all know substantial progress has been made (kudos goes to Vadim, Christian and Clayton + other RH folks on MCO team) and so everyone can already give it a try  and help providing feedback by raising issues on same repo  (in the near future i think the desire is to do it via BZ).
There are some hicups which Vadim is trying to fix it (many kudos to him) and so once that is done then we can share the news (with the OKD WG hat on).