Left: m = 1 mixed-mode mean frequency perturbation due to magnetism only (δν_mag in μHz) as a function of mixed-mode frequencies (ν_{1, g, max} in μHz) and the magnetic field amplitude in Gauss for a M_⋆ = 1.5 M_⊙, Z = 0.02. Typical limiting frequencies are indicated: black dashed lines represent the frequency resolution in the PSD corresponding to 4 years of Kepler continuous observations (∼8 nHz), 3 years of PLATO two main fields (∼11 nHz), 1 year of TESS continuous viewing zone data (∼30 nHz), a typical line width of radial modes, which are an upper limit for the line width of dipolar mixed modes (∼0.12 μHz), 2 months of PLATO stare-and-step observations (0.19 μHz), and 27 days of TESS individual sectors (0.37 μHz). The coloured area is limited by the critical magnetic-field amplitude range for the validity of the perturbative analysis. We choose to limit the results at B_c, g/1000, as defined in Sect. 3.5. Red lines represent the expected magnetic-field amplitude by considering magnetic-flux conservation from the end of the MS, with original fossil field amplitudes of 0.1 and 1 MG, as detailed in Sect. 2.3. Right: same as left panel, but simplified with the colour map replaced by typical limiting frequency values corresponding to Kepler data. The yellow area corresponds to the combination of ν_{1, g, max} and magnetic amplitude for which magnetic splittings cannot be visible in Kepler observations. The blue area corresponds to small magnetic shifts that should be visible in Kepler observations. The purple area corresponds to very large magnetic shifts, larger than the typical line width of radial modes.