Currently, rubber wood planks can't make fences, fence gates, doors, trapdoors, or a boat like other vanilla woods in 1.12. Could these be added?

Full list:

• Rubber log
• Rubber plank
• Rubber button
• Rubber door
• Rubber fence
• Rubber fence gate
• Rubber pressure plate
• Rubber slab
• Rubber stair
• Rubber trap door
• Rubber boat

Anything else?

I assume that development is exclusively for 1.14 now, but is it possible for these to be back ported to 1.12?

Little to no chance for backport. Also boat won't be implemented until texture available.