This post has a long intro, but please bear with me...
Back in 2018, after the then-Republican controlled Congress zeroed out the ACA's federal "individual mandate penalty" (officially the "shared responsibility penalty"), I posted both a video and slideshow explainer about what this penalty was and why it was included in the ACA in the first place.
In the United States, major medical insurance policies for those who don't have healthcare coverage through their employer, Medicare, Medicaid, CHIP, the Veteran's Administration or some other source are available via the ACA's individual market exchanges. The individual market for residents of 36 states is HealthCare.Gov; the remaining 14 states + DC each have their own ACA exchange, such as Covered California, NY State of Health and so forth.
There are usually dozens of ACA policies available via the ACA exchanges, but they fall into five major categories: Catastrophic, Bronze, Silver, Gold and Platinum plans (other major distinctions include HMOs vs. PPOs and other variables,but those are for another day).
With rare exceptions, Catastrophic plans are only available to enrollees under 30 years old. ACA premium subsidies can't be used to help pay for Catastrophic plans either, so enrollment is rare; during the 2020 Open Enrollment Period, only 89,000 ACA exchange enrollees selected Catastrophic plans out of over 11.4 million total, or just 0.8%.