That is quite literally illegal where I live. Any annex to the contract (including base salary changes) must be _voluntarily_ signed by the employee. Forcing signing is illegal.

When covid started, my company gave all employees annexes to reduce our pay by 20%, but keep our workload the same. I told 2 bosses, the HR employee and the director of HR mobbing me I will not sign and to piss off. I was the only one to do so out of ~150 employees. I kept my full salary until I quit my job because I don't like working for cunts.