> things like employer matches in retirement accounts

seems completely reasonable to model this as income.

> health insurance contributions

If your employer provides a health insurance benefit that would otherwise cost you $X / yr, you could model that as +$X / yr income and +$X / yr expenses.

Simple FIRE calculators like that Networthify one assume that your expenses in retirement are the same as when you are working, but that's probably not true for many people, for numerous reasons: health insurance once you're not longer working, paying off a mortgage, children launching into the workforce, increasing health expenses as you age, lifestyle changes when you're not working, potentially moving to a different area with a different cost of living.

But as a 0-th order approximation, assuming expenses in retirement are the same as when you are working is a place to start when assembling a rough plan.