  We presented 15 new T dwarfs that were selected from UKIRT Infrared Deep Sky
Survey, Visible and Infrared Survey Telescope for Astronomy, and Wide-field
Infrared Survey Explorer surveys, and confirmed with optical to near infrared
spectra obtained with the Very Large Telescope and the Gran Telescopio
Canarias. One of these new T dwarfs is mildly metal-poor with slightly
suppressed $K$-band flux. We presented a new X-shooter spectrum of a known
benchmark sdT5.5 subdwarf, HIP 73786B. To better understand observational
properties of brown dwarfs, we discussed transition zones (mass ranges) with
low-rate hydrogen, lithium, and deuterium burning in brown dwarf population.
The hydrogen burning transition zone is also the substellar transition zone
that separates very low-mass stars, transitional, and degenerate brown dwarfs.
Transitional brown dwarfs have been discussed in previous works of the Primeval
series. Degenerate brown dwarfs without hydrogen fusion are the majority of
brown dwarfs. Metal-poor degenerate brown dwarfs of the Galactic thick disc and
halo have become T5+ subdwarfs. We selected 41 T5+ subdwarfs from the
literature by their suppressed $K$-band flux. We studied the spectral-type -
colour correlations, spectral-type - absolute magnitude correlations,
colour-colour plots, and HR diagrams of T5+ subdwarfs, in comparison to these
of L-T dwarfs and L subdwarfs. We discussed the T5+ subdwarf discovery
capability of deep sky surveys in the 2020s.
