Well, now I am complaining.
Apparently the root cause of this is that DOTS (or at least the part of the stack that depends on the Entities package, which is most of DOTS) is not, and will not be, supported on Unity 2021. Per an announcement made yesterday , anything dependent on the Entities package will only be supported on Unity 2020 LTS. This will remain the case until some undefined time in the future, but definitely not before 2022.
So, for anyone who wants to work with the ECS part of DOTS (which is a core part of DOTS), you’ll need to stay on the 2020 LTS path until further notice. If you’re like me and you’re already running on the 2021 path, you’ll need to downgrade your project(s) to 2020 LTS in order to work with ECS.