Lepton universality violations are not seen in W boson decays at the LHC, are not found in tau lepton decays or pion decays (also here), and are not found in anti-B meson and D* meson decays or in Z boson decays. There were still tensions in the data from B meson decays, but the deviations were smaller as of 2019 than they were in 2015 (also here).
The critical point is that in a semileptonic decay of a b quark, the leptons arise from one or two subsequent leptonic W boson decays. So, if W boson decays don't display violations of lepton universality, then either the leptons observed in semileptonic b quark decays are produced by some means other than an intermediate W boson decay, exclusive to b meson decays, which decays preferentially to lighter leptons, or there is some error in the observation. The latter result is infinitely more likely.