Is eSIM on the iPad Air secure to use?

They're actually more secure than a SIM card since no one can remove it.

You could look this kind of information up yourself, but here's a few.

https://www.mintmobile.com/blog/esim-vs-physical-sim-cards-what-you-need-to-know/

https://www.betterroaming.com/blog/esim-data-plans/esim-vs-physical-sim/

https://www.croma.com/unboxed/esim-vs-physical-sim-making-the-right-choice-for-your-device#:~:text=Security%3A%20The%20biggest%20benefit%20of,it%20can't%20be%20tracked.

Both have their advantages and disadvantages, but overall, eSIMs are the better choice.