Legacy parts
Legacy parts are parts that are hidden in the VAB and SPH because these were replaced by new/better parts. Legacy parts are only available in old saves.
Command Modules
The Mk1-2 Command Pod was hidden in 1.4 in favor for the Mk1-3 Command Pod
Fuel Tanks
The ROUND-8 Toroidal Fuel Tank was hidden in 1.4 in favor for the R-12 'Doughnut' External Tank
Decouplers
The TR-2V Stack Decoupler was hidden in 1.4 in favor for the TD-06 Decoupler
The TR-18A Stack Decoupler was hidden in 1.4 in favor for the TD-12 Decoupler
The Rockomax Brand Decoupler was hidden in 1.4 in favor for the TD-25 Decoupler
The TR-38-D was hidden in 1.4 in favor for the TD-37 Decoupler
Separators
The TR-2C Stack Separator was hidden in 1.4 in favor for the TS-06 Stack Separator
The TR-18D Stack Separator was hidden in 1.4 in favor for the TS-12 Stack Separator
The TR-XL Stack Separator was hidden in 1.4 in favor for the TS-25 Stack Separator