I recently quitted a startup that I spent many years at. I was the first employee as engineer, and I own equity (not option). The management team has changed drastically over the years, and the tech that enabled the business is now considered a cost. The politics got overwhelming, which is why I quitted. The startup has made a few private rounds and added many members to the board. The management does not like tech at all. They offered a price for my shares, which I said no because of the low offer. And I received a message from the management saying my shares are restricted and subject to board cancelation.
I cannot tell if it is an unhappy/unreasonable threat message from the management or if it can actually be possible. In what scenario can a company legally remove your equity?
Technically shares represent ownership of a part (often very small) of the company. Depending upon the class of shares you might have a certain number of votes in the running of the company.
As long as you are recorded as owning those shares "fully paid up", they cannot be taken away from you. That would be considered to be theft.
It has nothing to do with any contract. Shares are transacted as if they were physical objects or title deeds to property, etc.
If you didn't actually pay for those shares and you have no share certificates, then things can get murky. As a shareholder, you are entitled to access the share register and verify the entries.
The above is based upon my practical experience, but I'm not a lawyer, nor accountant. As others have suggested if there is significant money involved then you need qualified legal representation.