You can copy textures from previous version (v20.0) and replace placeholder textures of deprecated parts.
For parts themselves just delete line "TechHidden = True" and assign some category to them in part *.cfg files, for example "category = Utility".
I restored few parts like this and from what I tested, they are fully functional. For parts with crew you will also likely need to assign some appropriate internal spaces.
So if anyone misses some older parts, you can just go through "DEPRECATED" folder and restore them