I'm UK native and had never heard of the word as an insult. I wonder if it's a regional thing?

Perhaps a generational (and maybe class?) thing as well; it and related terms were vicious and influential enough to have motivated The Spastics Society to change their name to Scope in the mid-nineties [1]; I guess growing awareness of disability discrimination and bigotry also contributed to it being recognized as beyond the pale and disappearance from casual conversation with the corollary effect of some people outside those demographics being ignorant of how awful those terms were.

[1] https://www.bbc.com/news/blogs-ouch-26788607

Well the UK has done an amazing job at getting rid of almost all uses of the word "spastic" due to the campaign against it, so it could be for that reason

I'm old enough to remember that and some other derivations being very common in the playground. Just not this one.

To be fair I didn't remember it until I put the "a" on the end, like "spacka". Then I remembered