I've been looking all over the net for an answer, but haven't been able to get it.

MtGO has sets specifically made for online play only, but because the Gatherer page doesn't really distinguish them from their paper cousins, as such they are listed as legal in Modern. Thus my confusion: is it's legality global? Can I grab an earlier copy that would have been illegal otherwise for paper?

If an example is needed, take Spike Feeder. It is listed as Modern legal, but it's real versions are all in sets that aren't modern. So...

SinkingStar wrote:If an example is needed, take Spike Feeder. It is listed as Modern legal, but it's real versions are all in sets that aren't modern. So...

Spike feeder is legal in modern because it was Time Shifted in the Time Spiral set.

Not sure about much else but I assume that they remain legal based on their paper printings.

If a card was ever printed in a modern legal set, any printing of that card may be used. For example, Lightning Bolt has been printed in a few modern sets. One could use Alpha bolts if they wanted to.