  We present a search for associated HI 21 cm absorption in a sample of 29
radio-loud active galactic nuclei (AGNs) at $0.7 < z < 1$, carried out with the
upgraded Giant Metrewave Radio Telescope. We detect HI 21 cm absorption against
none of our target AGNs, obtaining $3\sigma$ upper limits to the optical depth
of $\lesssim$ 1% per 50 km s$^{-1}$ channel. The radio luminosity of our
sources is lower than that of most AGNs searched for HI 21 cm absorption at
similar redshifts in the literature, and, for all targets except two, the UV
luminosity is below the threshold $10^{23}$ W Hz$^{-1}$, above which the HI in
the AGN environment has been suggested to be completely ionised. We stacked the
HI spectra to obtain a more stringent limit of $\approx 0.17$% per 50 km
s$^{-1}$ channel on the average HI 21 cm optical depth of the sample. The
sample is dominated by extended radio sources, 24 of which are extended on
scales of tens of kiloparsecs. Including similar extended sources at $0.7 < z <
1.0$ from the literature, and comparing with a low-$z$ sample of extended radio
sources, we find statistically significant ($\approx 3\sigma$) evidence that
the strength of HI 21 cm absorption towards extended radio sources is weaker at
$0.7<z<1.0$ than at $z < 0.25$, with a lower detection rate of HI 21 cm
absorption at $0.7 < z < 1.0$. Redshift evolution in the physical conditions of
HI is the likely cause of the weaker associated HI 21 cm absorption at high
redshifts, due to either a low HI column density or a high spin temperature in
high-$z$ AGN environments.
