I hadn’t been aware of this 2019 paper in Legislative Studies Quarterly by David Cotrell. The piece itself is behind a paywall. Here’s the abstract:
Recent research has leveraged computer simulations to identify the effect of gerrymandering on partisan bias in U.S. legislatures. As a result of this method, researchers are able to distinguish between the intentional partisan bias caused by gerrymandering and the natural partisan bias that stems from the geographic sorting of partisan voters. However, this research has yet to explore the effect of gerrymandering on other biases like reduced electoral competition and incumbency protection. Using a computer algorithm to design a set of districts without political intent, I measure the extent to which the current districts have been gerrymandered to produce safer seats in Congress. I find that gerrymandering only has a minor effect on the average district, but does produce a number of safe seats for both Democrats and Republicans. Moreover, these safe seats tend to be located in states where a single party controls the districting process.
Update: Here is the bottom-line result: “In fact, around 36 marginally Democratic seats are replaced with approximately 18 safe Republican seats and 18 safe Democratic seats as a result of gerrymandering.”
Put another way, in districting done without regard to political outcomes, there would be around another 36 competitive seats. That would increase the number of competitive seats by about 50%.
Note that this is the result without putting any weight on competitiveness in the design of districts.