FYI the US is not withdrawing from 66 "International Organizations, Conventions, and Treaties" - this is a dishonest framing by the Trump administration. Most of the entities listed in the memorandum of 7 Jan 2026 are only commissions, coordination mechanisms and sub-organs of international organizations. A few of them are quite important (e.g. International Law Commission) and many do good work, but most of them are not international organizations.
For example, you have the United Nations (the international organization) which has an organ called the Economic and Social Council (ECOSOC) , which created a commission called the "Economic Commission for Africa". The US is only withdrawing from the commission (and 3 other regional commissions), not ECOSOC or the UN as a whole.
Why does this matter? There are not that many international organizations in the world and withdrawing is a major step with long-term consequences. By contrast there are many, many thousands of mechanisms, commissions and sub-organs. Withdrawing from 66 of these will certainly do some damage, in particular because the withdrawal focuses on entities working on climate change, democracy, the rule of law, disarmament, peace and humanitarian issues. However, this is an entirely different impact on the international order than withdrawing from the UN or the WTO, for example.