If it is no longer common to do this, you have the potential to stand out all the more for performing this social courtesy.

This is not one of those things like wearing a suit to a shorts-and-sandals startup, where they say "Oh what a dweeb, he's totally a not a culture fit here." No one thinks badly of sending a simple thank you note. Don't write them a love letter or anything, but spend the 2 minutes to write a sincere custom note, and snail mail it if possible, or email it otherwise.