That's unexpected. Then again, 1.8B is a lot of money for what I'm guessing is a small number of major shareholders.
After seeing how Tencent purchased gaming companies have fared (that is, typically with few changes), this probably won't have much of a negative impact on the site.
I may be being a little more optimistic than some other people here, but I agree with you. There's a history of larger companies acquiring smaller ones only to ruin them (ahem, Yahoo), so hopefully Prosus understands what makes Stack Overflow so successful and will be mostly hands off. (Also, I'm not sure when Prosus acquired Udemy, but as far as I know that acquisition didn't negatively affect that site.)
After seeing how Tencent purchased gaming companies have fared (that is, typically with few changes), this probably won't have much of a negative impact on the site.