Due to Bamboo Rafts and Bamboo Rafts With Chest using the same entity IDs as Boats and Boats with Chest, you can use commands to summon a Bamboo Raft or Bamboo Raft With Chest even when 1.20 is disabled.

The entity can then be used, and can even be obtained as an item by either breaking it (in survival mode) or using pick block (in creative mode).

Steps to reproduce:

Method A - direct summoning

1. Open a world with 1.20 content disabled
2. Run /summon boat ~ ~ ~ {Type: "bamboo"} to summon a Bamboo Raft
   Note the same works for chest_boat to summon a Bamboo Raft with Chest

Method B - transformation

1. Open a world with 1.20 content disabled
2. Ensure there are no existing boats in the world
3. Place down a boat
4. Run /data merge entity @e[type=boat,limit=1] {Type: "bamboo"} to turn it into a Bamboo Raft
   Note the same works for type=chest_boat to turn it into a Bamboo Raft with Chest

Method C - spawn egg

1. Open a world with 1.20 content disabled
2. Run /give @s bee_spawn_egg{EntityTag: {id:"boat", Type: "bamboo"}} to receive a spawn egg
   Note the same works with any spawn egg, and with id:"chest_boat" to spawn a Bamboo Raft with Chest
3. Place the spawn egg to get a Bamboo Raft