My company is currently in a process of phasing out RHEL. Therefore, I was really looking forward to Connections v7, to maybe see support for CentOS8. But now RedHat/IBM announced a sudden EOL for CentOS 8 by the end of 2021. (https://blog.centos.org/2020/12/future-is-centos-stream/)
Which OS should I choose now? Windows is not an option for us, and as an alternative, there is only support for an older SLES version listed.
What about support for openSuse or Ubuntu or something else that doesn't cost the world?
In my personal opinion (because the announcement is just too new) I would wait for https://rockylinux.org/ which looks promising and is founded by the former CentOS founder.
Doing any kind of your preferred Linux (wouldn't do any kind of Kubernetes with Windows) for the "pink" or ComponentPack stack should work with the new Kubernetes support statement of Connections 7.
So for the legacy part you're dependent from IBM and HCL (I had the same issue with Windows Server 2019), that WebSphere 8.5.5 and DB2 11.1 is not the newest product and does not support a lot of up2date OS.
Websphere:
https://www.ibm.com/software/reports/compatibility/clarity-reports/report/html/softwareReqsForProduct?deliverableId=1345529544358&osPlatform=Linux (and HCL doesn't support all of them). - RHEL 8, SLES 15, Ubuntu 18.04
DB2:
https://www.ibm.com/software/reports/compatibility/clarity-reports/report/html/softwareReqsForProduct?deliverableId=C0871B109FDF11E389FAED6C8DACE45F&osPlatform=Linux - RHEL 7, SLES 12 and Ubuntu 20.04
TDI/SDI -> could be installed on an older OS, because it does not need a lot of resources.
I don't expect an official support statement of HCL supporting Ubuntu, but maybe when enough people ask for it? On the other side, most customers need or want official support of a manufacturer.
Now to your main question: when you want to start in the next weeks, I would go for CentOS 7 and build as much as you can with Terraform and Ansible (the roles which HCL released yesterday look promising). So you can rebuild the environment end of 2021 to a supported OS in the same manner.