Almost every programming problem I knew of back in the 1990s is now solved. The main exception, "distributed computing is really complicated and nobody has any idea how to make it work reliably," is now everybody's day job. Although defining "reliably" turned out to be harder than we thought.
Relatedly, percentiles were not too useful to me back in the 1990s, and seemed kinda contrived. Nowadays they're surprisingly essential.
ssh+2FA to all your machines, anywhere, without opening firewall ports.