I think this is very common playbook those days - Overdeliver to your customers whenever with Free Tier, Everything Open Source, Extremely Low Prices to get market share... when do a switch and focus on profitability. With killing free tier I wonder if it just was not generating a good conversion to paid users or is it killing a golden goose to get one time customer injection from folks choosing to move to paid tier
At the end of the day, it's not cheap (and certainly not free) to maintain databases and infrastructure, especially with the additional features PlanetScale has.