Primeval very low-mass stars and brown dwarfs - V. A halo L3 subdwarf with prograde eccentric orbit in the Galactic plane

VVV J12564163$-$6202039 is an L subdwarf located in the Galactic plane ($b$ = 0.831 deg) discovered by its high proper motion (1.1 arcsec yr/s). We obtained an optical to near-infrared spectrum of it with the X-shooter on the Very Large Telescope, and re-classified it as an sdL3 subdwarf. Its best-fitting BT-Dusty model spectrum has $T_{\rm eff}$ = 2220 K, [Fe/H] = $-0.9$, and log $g$ = 5.5. It is just below the stellar/substellar boundary in the $T_{\rm eff}$ versus [Fe/H] parameter space. This object is at a distance of 66.94$^{+8.49}_{-6.77}$ pc according to the $Gaia$ astrometry. It has a halo membership probability of 99.82 per cent according to its Galactic kinematics. It has unusually flat (|$Z$| < 500 pc) and extremely eccentric (e = 0.9) prograde orbit that takes the source from as close as 1.2 kpc from the Galactic centre to 24 kpc out. VVV J12564163$-$6202039 joined by SDSS J133348.24+273508.8 and ULAS J021258.08+064115.9 are the only three known objects in the sdL subclass that have halo kinematics.