The differentiation in check/test pipeline proves to be another barrier new developers are facing. It is/was necessary because we run/ran tests on permanent instances and didn't want anyone to be able to execute just any code there. This isn't a concern for one-off instances that are destroyed after each test was finished. As I understand it getting rid of it this differentiation has been a goal for quite a while and there's no doubt we want to get there.
This goal was described and tracked in T47499, which got closed because the way chosen to reach this goal was Nodepool. The job of tracking the effort to get everything into one-off instances (Nodepool) was then taken over by Continuous-Integration-Scaling and it's tasks (e.g. T119138, T134381, T119140) . These were declined and the project was archived, because nodepool is now considered legacy and the plan is to have docker for everything. There is T190097: Migrate all CI jobs from Nodepool, deprecate its use, but I fail to find a project or task "Migrate all CI jobs from permanent instances".
I wonder, but cannot easily answer the questions which jobs (or whether any at all) we still run on permanent instances, if the check/test differentiation is still needed, and if it is, if we're seeing progress in a 'permanent instances -> docker' transition, and how long the whitelist will continue to be necessary.