Honestly, I think you could create your own pretty easy using a simpler version of what Kentico v12 and earlier offered. I have a client who uses forums VERY heavily and they are stopping at version 12 due to no support in v13. After they decide they've waited long enough, we'll review Kentico Xperience or move to another platform.
If you're not up for building one or using v12, then check out this Github search, there are quite a few open source options to check out!