I have opened an R20 project file with a fresh version of the R20 House Builder in R21: It is not working. But any modification that I did in R20 will open in R21. (Based on a quick test)
I hope it is part of the Content Browser soon again.
Apologies if this has been discussed elsewhere, but I believe the old House Builder tool that’s available from Nitro4D is called NitroRoom. You can get it at: https://nitro4d.com/product/nitroroom/