Modifies provisions regarding income taxes, including the imposition of a 4% flat income tax

SB 5 replaces Missouri's current progressive income tax system with a flat 4% tax on all taxable income for residents starting in 2026. For tax years 2023-2025, it temporarily caps the top tax rate at 4.95% (down from the current 6% rate), with potential future reductions of 0.15% annually (starting 2024) or 0.1% annually (starting 2027) if state revenue exceeds specific inflation-adjusted thresholds. The bill directly affects all Missouri residents who file state income tax returns. Key mechanisms include the transition period to a flat tax, revenue-based reduction triggers, and automatic adjustments to tax tables by the state revenue department.