Thanks for getting in touch! Great question; we bumped the minimum OS requirement to Windows 2012 R2 in Octopus version 2020.x and wrote a blog post at the time discussing this (and SQL server minimum requirement bump as well) in some more detail here.
You should be okay to upgrade to v2019 on your 2008 R2 server, however we would still recommend upgrading your OS and Octopus to get the features, bug fixes, security patches, etc.
I hope that helps, and please let us know if you have any further questions going forward!
Thanks @Kenneth_Bates ! Yes, we have the plan to do major overhaul upgrade of the build box to latest windows server version, and this is interim solution.