I'm confused about the "Timeshifted" cards. I looked at the Wizards of the Coast rarity scale. How rare are they and are they valid in Modern? (example Undertaker)
Timeshifted cards are (slightly more than) 1.5 times as rare as Rare cards from Time Spiral. There were 80 Rare cards and 121 Timeshifted cards in the set, and each booster pack included 1 Rare card and 1 Timeshifted card.
Due to the rarity distributions, each Mythic Rare in a large set appears in 1 out of every 121 boosters, meaning that they are exactly as rare as Timeshifted cards!
Despite using the old borders, Timeshifted cards are legal in Modern, since they were printed in Time Spiral, which is a Modern-legal set.
Every Timeshifted card is a reprint of a card from a set prior to 8th Edition.
The Planar Chaos and Future Sight sets had their own versions of Timeshifted cards, but they did not have their own rarity.
Timeshifted were rather special. Every booster pack from the Time Spiral block had one Timeshifted card, and each Timeshifted card was equally likely to be the included card.
The Timeshifted cards are distributed one per booster pack and three per tournament pack (taking the place of common cards), and also appear in the preconstructed decks. According to DCI-distributed tournament primers, Timeshifted cards are tournament legal where Time Spiral or the set of original printing are.
Rarity has nothing to do with legality. The timeshifted expansion symbol color just indicates that the card is a reprint in the Time Spiral set.
Legality in different formats depends on what sets a card was printed in. Undertaker, for example, was printed in Mercadian Masques and Time Spiral, so any version of Undertaker is legal in any format that allows either Mercadian Masques or Time Spiral.
The modern format lists Time Spiral, Planar Chaos, and Future Sight as legal sets, so any cards from those sets (other than specifically banned cards), including timeshifted cards, are legal.
As ikegami says in his answer, each Time Spiral pack contained exactly one timeshifted card, so they were about as common as rare cards before mythic rares were introduced.