Most of Trump's policy work has been on the regulatory side of things. Environmental policy is one of my biggest areas of interest, and the administration has been very active in that area. This isn't a comprehensive list, but here are some of the worst environmental policies that have been enacted since 2017 IMO:
Shrank Bear Ears National Monument and Grand Staircase Escalante to allow for mining, drilling, and grazing. Bear Ears was shrunk by 86% and Grand Staircase was reduced by 46%, the largest rollback of public land protections in US history.
Repealed the Methane and Waste Prevention Rule, which regulated the waste of natural gas from venting, flaring, and leaks during oil and natural gas production.
Rolled back the National Environmental Protection Act, eliminating the requirement for agencies to consider greenhouse gas emissions during environmental assessments.
Revised the Mercury and Air Toxic Standards, allowing for more air pollution than under previous rules.
Withdrew CAFE fuel economy standards, removing automakers' responsibility to improve fuel mileage in new cars and trucks.
Rolled back Refrigerant Management regulations, narrowing the class of refrigerants that are prohibited from being knowingly vented or released while repairing/disposing of appliances.
Halted implementation of the Accidental Chemical Release Prevention Rule, which was meant to improve safety at facilities that use hazardous chemicals. This rule was proposed in 2013 after the explosion at a Texas fertilizer facility.
Rescinded the Clean Fuels Grant Program, which assisted the development of clean energy technologies.
Repealed light bulb energy conservation standards, reducing the number of light bulbs subject to energy efficiency regulation
Rescinded the Clean Power Plan, which required coal-fired power plants to reduce carbon emissions.
Reversed a ban on chlorphyrifos, a pesticide that has been linked to brain damage in children and adults.
Scaled back Oil Well Control Rules, which set standards to prevent blowouts during oil drilling.
Withdrew Compensatory Mitigation component of the Endangered Species Act, which required developers to compensate for the impacts of their activities on public lands.
Rescinded greenhouse gas standards for the transmission and storage sectors, expected to increase methane emissions by 350k-370k tons and save the oil & gas industry $97MM-$123MM.
Repealed Greenhouse Gas Emissions Measures which required state departments of transportation to track and reduce carbon emissions on the national highway system.
Rescinded Oil and Gas Fracking Rules which required disclosure of chemicals and other details of fracking operations.
Permitted Oil drilling in the Arctic National Wildlife Refuge, making the entire area available for oil and gas leasing