How much extra work is it to put new flavor text on a reprint? For example, Necropolis Regent has Ravnica specific flavor text, and it would be cool if the Explorers of Ixalan and Crimson Vow Commander versions had plane-specific flavor text. I realize its a pretty low priority (and probably a big ask for every card in a product) but it would be cool.

Reprints in a premier set usually get new flavor text. Reprints in a supplemental set, especially a mostly reprint set, less often get new flavor text.

In your article, you said Tombstalker wasn't printed because there are no non-cat demons on Tarkir, but what about Necropolis Fiend?

What actually happened was at the time of the decision to remove Tombstalker there were zero demons. Then later in development Erik talked with the creative team about how we were a little low on iconics and the decision was made to add demons but no one at the time thought about the earlier decision to not do Tombstalker so it never got revisited.