Add deprecation notes to function deprecated in 2021

Add deprecation notes to function deprecated in 2021

I looked to remove this but the original PR spotted it was used
by civimobile & I checked & it still is - albeit in code
that is deprecated within civimobile. I created an issue on their
repo & updated the notes on the function to point to it & also
suggest that we remove it around 5.66 (which gives another 3-4 months
for civimobile to respond now there is an active issue not
just a notice)