Safety data sheets for all the Ilford products are available here:
https://www.ilfordphoto.com/health-and-safety/safety-data-sheets/english-australian-msds/
In my practical experience, fixer washes quite easily off smooth surfaces - unless it has been allowed to dry, when it seems impossible to shift. However, it sticks rather well to textured surfaces like human skin and wood. So in a darkroom, hands can be a significant source of contamination. Smell is the best warning: you quickly learn to recognise the smell of fixer on your hands. For the same reason I've never understood why some people favour wooden tongs.
Next time dump it in a toilet or even better find a minilab that will take it.
As has been stated above, the dissolved silver kills aquatic organisms and microorganisms including those in septic tanks. If you only chuck an occasional developing tank full, the consequences will of course be trivial, but bear it in mind if you scale up.
When I was a kid venturing into photography, my only option was to develop films at the kitchen sink. My mother ruled the kitchen absolutely, so this was a major concession. When I had finished, she returned to her Empire and blitzed everything with a foul preparation called Jeyes Fluid, which was supposed to kill every microorganism known to man and otherwise. I could never persuade her that the JF was infinitely more of a concern.
