Content size

[Awestruck]

We can't do much about game content as it gets taken down from the workshop. People need to either buy the games or download the content for them from elsewhere themselves.

Indeed, but that aspect must be acknowledged to prevent it becoming a problem in the first place, for example the mining rocks are from CS:S, that means everyone that has not installed CSS sees errors. That is a huge percentage of people and the models should be replaced.

For a long while, having CS:S & Episodic content has sort of been an unofficial requirement for players. If a new player purchases Gmod, then sooner or later they will run into maps and servers that require the CS:S/Episodic content, and there isn't much that can be done about it. CS:S/Episodic content has become necessary for most things in the game, and they are especially necessary in the City RP gamemodes. 

Disabling/replacing the content would be quite impossible, as they play a huge role as either map models or spawnable props. I can't really blame the mappers for it, as the base HL2 textures and models are simply horrendous looking and most of the time they won't fit at all in a modern city setting. Disabling CS:S props would put a massive limit on what you can and cannot build in the game, and finding a good city map that doesn't use that content is quite hard as well.

Why is it not possible to at least provide a source to the content whenever players join the server similar to how new players get a hello/help message when they join? So it provides a link to obtain the content so they don't just leave?

It would be a very useful feature or at least to have it pinned on Discord/forums help & support section, though technically speaking it does count as piracy, even though you won't get punished for it. Valve doesn't support it and I believe those links are not allowed on Steam either. So it's kind of a gray area, which is why when someone posts a link it might stay or sometimes it might get removed.

Siphon is correct. It is an act of piracy for FL to advertise bootleg copies of paid content. Users are able to find the content online (undoubtedly not endorsed by FL to do so), or buy it.