In Octopus Deploy, each environment as a setting called Dynamic Infrasture. There is a checkbox to 'Allow dynamic infrastructure', and I have it turned on.
How can I turn this off at the project level?
I have one project that needs this off, but older projects need it on.
It is not currently possible to turn that feature off for a single project.
You could move the project to a different space, or create a new set of environments with a new lifecycle for that project only.
The Dynamic Infrastructure flag only affects how the deployment will accept create target commands. If you aren't using those then it has no effect.