Is there such a thing as a "Waiver of Annual Leave" these days??

‌I am currently working on a contract basis.
I haven't been able to use all my annual leave.
My contract is nearing its end,
and it looks like I won't be able to use the remaining days.
If I try to use them now, the company is too busy,
so when I mentioned using my leave, they told me not to.
When I asked if I would be compensated for them in cash instead,
they said no, and that I should just sign a "Waiver of Annual Leave" lolololol
In all my life, I've never heard anything like this lolololololol
I was so stunned by how ridiculous this sounded
that I just froze for a moment.
I'm not an expert, but aren't those kinds of waivers illegal?? lol
Even if I sign it, what's the point?
I mean, it's not like the company is a gang or something, why on earth would I have to sign a waiver like that?