Tax liabilities on (optional) employee secondary stock sales is most certainly not a normal business operation.
GAAP is a set of accounting standards - it doesn't require taking actions, it's about how to record things if you want to meet those standards. So no, there is no GAAP requirement to build a cash reserve. For what it's worth, Stripe isn't even obliged to follow GAAP for their accounting (it is likely they signed a contract somewhere which calls for a set of financials meeting GAAP to be furnished periodically, but again GAAP doesn't require actions, it stipulates accounting entries).