I can certainly think of a few good and rare old ones. Harvestman Double Andore 1 and Hertz Donut 1 are absolutely fantastic modules that are difficult to get a hold of nowadays. The Livewire AFG was a staple module when I got started in 2010 as well as the Toppobrillo Sport Modulator, both now obscure and hard to find. Of course the elusive QMMG. :-)

On the topic of what 'modern' modules are becoming rare, I figure the Sputnik modules will all essentially be hard to find in the future. They are all very good, and as far as I know all production has seized. Maybe the Roland Demora/Scooper et al are candidates too?