We report upper critical field B_c2(T) data for LaO_0.9F_0.1FeAs_1–δ in a wide temperature and field range up to 60 T. The large slope of B_c2 ≈ –5.4 to –6.6 T K^–1 near an improved T_c ≈ 28.5 K of the in-plane B_c2(T) contrasts with a flattening starting near 23 K above 30 T we regard as the onset of Pauli-limited behaviour (PLB) with B_c2(0) ≈ 63–68 T. We interpret a similar hitherto unexplained flattening of the B_c2(T) curves reported for at least three other disordered closely related systems, Co-doped BaFe₂As₂, (Ba, K) Fe₂As₂ and NdO_0.7F_0.3FeAs (all single crystals), for applied fields H ∥ (a, b), also as a manifestation of PLB. Their Maki parameters have been estimated by analysing their B_c2(T) data within the Werthamer–Helfand–Hohenberg approach. The pronounced PLB of (Ba, K)Fe₂As₂ single crystals obtained from an Sn flux is attributed also to a significant As deficiency detected by wavelength dispersive x-ray spectroscopy as reported by Ni et al (2008 Phys. Rev. B 78 014507). Consequences of our results are discussed in terms of disorder effects within conventional superconductivity (CSC) and unconventional superconductivity (USC). USC scenarios with nodes on individual Fermi surface sheets (FSS), e.g. p- and d-wave SC, can be discarded for our samples. The increase of dB_c2/dT|_Tc by sizeable disorder provides evidence for an important intraband (intra-FSS) contribution to the orbital upper critical field. We suggest that it can be ascribed either to an impurity-driven transition from s_± USC to CSC of an extended s₊₊-wave state or to a stabilized s_±-state provided As-vacancies cause predominantly strong intraband scattering in the unitary limit. We compare our results with B_c2 data from the literature, which often show no PLB for fields below 60–70 T probed so far. A novel disorder-related scenario of a complex interplay of SC with two different competing magnetic instabilities is suggested.