  Nuclear $\beta$ decays as well as the decay of the neutron are
well-established low-energy probes of physics beyond the Standard Model (SM).
In particular, with the axial-vector coupling of the nucleon $g_A$ determined
from lattice QCD, the comparison between experiment and SM prediction is
commonly used to derive constraints on right-handed currents. Further, in
addition to the CKM element $V_{us}$ from kaon decays, $V_{ud}$ from $\beta$
decays is a critical input for the test of CKM unitarity. Here, we point out
that the available information on $\beta$ decays can be re-interpreted as a
stringent test of lepton flavor universality (LFU). In fact, we find that the
ratio of $V_{us}$ from kaon decays over $V_{us}$ from $\beta$ decays (assuming
CKM unitarity) is extremely sensitive to LFU violation (LFUV) in
$W$-$\mu$-$\nu$ couplings thanks to a CKM enhancement by $(V_{ud}/V_{us})^2\sim
20$. From this perspective, recent hints for the violation of CKM unitarity can
be viewed as further evidence for LFUV, fitting into the existing picture
exhibited by semi-leptonic $B$ decays and the anomalous magnetic moments of
muon and electron. Finally, we comment on the future sensitivity that can be
reached with this LFU violating observable and discuss complementary probes of
LFU that may reach a similar level of precision, such as
$\Gamma(\pi\to\mu\nu)/\Gamma(\pi\to e\nu)$ at the PEN and PiENu experiments or
even direct measurements of $W\to\mu\nu$ at an FCC-ee.
