I also hope that in Openfire, there can be many domains, can this be done in 2021?
No need to revive a 13 years old post to ask a general question. Multiple domains support is not planned at any time. Developers think this is too complex to implement. https://igniterealtime.atlassian.net/browse/OF-162
I don’t see this happening, ever, unless someone is willing to invest an insane amount of time or resources - and even then, I’d prefer if those resources are spent on other stuff, as nowadays, it is relatively easy to have multiple instances of Openfire running on the same hardware (using containers / virtualization).
It is extremely complex to add multi-domain support as afterthought. That is true for all software: If it wasn’t designed with multi-domain support from the beginning, it requires an enormous amount of resources to implement afterwards.
So unless someone is willing to put in the resources, I don’t think that this will happen.