I believe most do though, but with a right of first refusal. In which case they will buy them of direct you to their preferred buyer. Source: I have done this and my conversation with the broker for the secondary market who handles a lot of this said it is quite common and that in fact other at the same company I was at had done so. It might be frowned upon internally but you have to look out for yourself. Staying at a startup for 8+ plus years isn't very realistic for me personally.

Many companies have periods of time where you get an opportunity to cash out a percentage of your equity. It gives confidence to early employees that the company has their back and they aren't really disincentivized to leave because they still have a majority of their equity "locked".