Short answer: Income doesn’t affect eligibility for employer or unsubsidized individual plans, but it determines eligibility for ACA premium tax credits, cost-sharing reductions, and Medicaid/CHIP.
Employer group coverage and unsubsidized individual coverage have no income test; anyone eligible can enroll regardless of earnings. Income matters only for assistance: it sets eligibility for ACA premium tax credits and cost-sharing reductions on the marketplace, and for Medicaid or CHIP. So while income never blocks you from buying coverage, it can determine how much help you get paying for it.
Sources
42 U.S.C. 300gg-1 (PHS Act §2702) — guaranteed availability: an issuer must accept every applicant, so there is no income test to enroll in coveragelaw.cornell.edu/uscode/text/42/300gg-126 U.S.C. 36B — the premium tax credit is available to an applicable taxpayer based on household income relative to the federal poverty linelaw.cornell.edu/uscode/text/26/36B42 U.S.C. 18071 — cost-sharing reductions are tied to the enrollee’s household incomelaw.cornell.edu/uscode/text/42/18071HealthCare.gov, Save on costs — how income determines marketplace savings and Medicaid/CHIP referralshealthcare.gov/lower-costs