promised to end payroll taxes, which funds Social Security and Medicare?

Trump announced payroll tax deferments on August 8, 2020 for financial relief amid the COVID-19 pandemic. He promised to make the cuts permanent if reelected.

If I’m victorious November 3rd, I plan to forgive these taxes and make permanent cuts to the payroll tax. I’m going to make them all permanent…I may extend and terminate. In other words, I’ll extend it beyond the end of the year and terminate the tax.

The following week, Trump reiterated that if reelected, he would terminate the payroll tax entirely.

And the payroll tax — we’ll be terminating the payroll tax after I, hopefully, get elected. We’ll be terminating the payroll tax, so that will mean anywhere from $5,000 to even more per family, and also great for businesses and great for jobs. A lot of people will be very happy to hear that. A lot of the great — certainly, conservative economists will be great to have — they think that’s the greatest thing we can do. That’s better than the payments; that’s better than anything else.

Payroll taxes are responsible for funding Social Security, a monthly stipend for senior citizens instituted in 1935. As of June 2020, 64 million Americans collected Social Security. Eliminating payroll taxes, which as of 2019 funded 89% of Social Security, could be an existential threat to the program. It also contributes to Medicare, which provides healthcare to some 60 million beneficiaries.

The majority of federal payroll taxes go towards funding Social Security

Trump denied that he would institute any cuts to the programs, saying that instead they would be paid for through the general fund, potentially adding their cost to the deficit.

Center on Budget and Policy Priorities – Policy Basics: Top Ten Facts about Social Security 
Social Security Administration – How is Social Security financed?
Associated Press – Trump’s suggestion to eliminate payroll tax doesn’t add up
CNN – Eliminating payroll tax could deplete Social Security by 2023, chief actuary warns
Forbes – Biden Claims Trump Has ‘Planned Cuts To Social Security’ By Killing The Payroll Tax. What Are The Facts?
USA Today – Fact check: President Trump has not said he will terminate Social Security