The issue is a world with multiple Dwarven civs. The Rainy Anvil-Bracelets has access to iron/steel but the Purple Oily Frost-Tunnel has access to iron but no flux (steel) and the Battle-Rock of Rainbows has neither. It's annoying to find a lack of iron and have to go back to the very beginning of the embark process.
That would throw a world gen rejection popup, I'm saying. ALL dwarves have to have iron for a vanilla world gen passing score.
So, automatically rejecting any world made in a total conversion mod? Automatic rejection after world gen already ran a hundred years or more, when all other rejections happen before the years start counting up?
Yup.
I think you might be misinterpreting "worldgen rejection popup" to mean "all of those things would be impossible." Maybe you don't do a lot of world gen customizing, but basically what it means is that it tries and rejects worlds that don't qualify some number of times, then brings up a popup window that tells you what's wrong.
You then have the option of saying "Abort and try to fix the parameters, OR IGNORE it and keep going."
So you can still play any of those mods... you just have to click ignore. And it's on your head if your dwarves don't have anvils, cause you're playing a weird mod. It's not Toady's job to accommodate that down to the level of making sure you have all the resources you personally want in your custom mod by default, etc...
For people in vanilla, there'd be little reason to ever click ignore / it wouldn't come up very often anyway. So they'd be guaranteed their anvils unless they explicitly agreed not to be, and then that's on them.