  We present radio observations of the most slowly rotating known radio pulsar
PSR J0250+5854. With a 23.5 s period, it is close, or even beyond, the
$P$-$\dot{P}$ diagram region thought to be occupied by active pulsars. The
simultaneous observations with FAST, the Chilbolton and Effelsberg LOFAR
international stations, and NenuFAR represent a five-fold increase in the
spectral coverage of this object, with the detections at 1250 MHz (FAST) and 57
MHz (NenuFAR) being the highest- and lowest-frequency published respectively to
date. We measure a flux density of $4\pm2$ $\mu$Jy at 1250 MHz and an
exceptionally steep spectral index of $-3.5^{+0.2}_{-1.5}$, with a turnover
below $\sim$95 MHz. In conjunction with observations of this pulsar with the
GBT and the LOFAR Core, we show that the intrinsic profile width increases
drastically towards higher frequencies, contrary to the predictions of
conventional radius-to-frequency mapping. We examine polarimetric data from
FAST and the LOFAR Core and conclude that its polar cap radio emission is
produced at an absolute height of several hundreds of kilometres around 1.5
GHz, similar to other rotation-powered pulsars across the population. Its beam
is significantly underfilled at lower frequencies, or it narrows because of the
disappearance of conal outriders. Finally, the results for PSR J0250+5854 and
other slowly spinning rotation-powered pulsars are contrasted with the
radio-detected magnetars. We conclude that magnetars have intrinsically wider
radio beams than the slow rotation-powered pulsars, and that consequently the
latter's lower beaming fraction is what makes objects such as PSR J0250+5854 so
scarce.
