We used to have player run servers for years. Is it some lost skill to write software that way?

It's not a lost skill.

Spinning up a binary and replicating actors across two computers that both have a connection string to a server is.. for all intents and purposes: easy.

Where it falls down is when you start to have complex interactions with AI that's serverside, or you have a dynamic world that changes based on player behaviour, or you have cross platform requirements, integrations with companion apps and above all: matchmaking.

If you're a looter-shooter, there's a whole host of complicated interactions too.

A game like Apex Legends could probably distribute their server binary, but if you require online, as in, not just a single match, but an economy- a dynamic matchmaker and a dynamic world (meaning: when you kick a box it stays kicked) and a persistent account (you keep your loot): then that doesn't work well anymore.

The interactions are just too complex to batten down reliably, they'll be exploited, there'll be no fun, or: it just won't be possible for certain features, regardless of safety.

You can see how this looks by trying to use one of the many unofficial versions of Runescape.