In my experience, as a normal non-unicorn employee at a large corp, I was able to have my contract modified. It required advice from an employment law lawyer that cost a few hundred dollars and a couple emails with the company’s general counsel.

You can change these contracts. Hiring people is difficult and once the company has made that decision they don’t want to lose you over a contract clause.

Some companies are literally legally barred from individualising employment contracts due to active union or government contracts they have.