Is new designer removed?
No, new designer will gradually replace classic designer.
but I can't see the new designer button now, so is it temporary hidden? Is this expected that new designer button is missing?
My flow doesn't compatible with classic designer, a lot of step is not show.
You can consider editing the flow directly in the make.powerautomate portal, and be sure to select the environment that is consistent with your app.
I tried edit in make.powerautomate, but result is same, a lot of step is not show, I did not change my environment.
Two days ago, I can see those step in run history with new designer mode. Now the run history can only view with classic designer mode too...