There are a couple of issues here.
First, if you import Observable from rxjs/Observable, you need to import individual operators. If you import Observable from rxjs (which you may have been doing before), all the operators are already available to you (at the cost of app binary bloat).
A second issue involves lazily loaded pages. If you import (for example) map in lazy page A, it won’t be available in lazy page B. There is a kludgy workaround that may or may not continue to work that involves importing it in app.component.ts. See this discussion for more details.