How exactly it should works?

Commands "/command" should be recognized and executed by client not by server.

as per MUC discussion: this should be a simple "drop in" solution given that there is no xep specification for history retrieval nor any support on the client side; basically upon receiving command "/loadhistory x" MUC component should send to the user x latest messages.

Yes, looks like this kind of /command stuff would be the most simple to add, not affecting client side software, so it would work with any client, and then it does not break anything on the server side if the server does not recognizes it.

This is actually a nice idea for adding some useful stuff on the server in general. I mean for normal users in other parts of the server. In theory any Tigase plugin could have a bunch of such commands available.

As I said: it cannot be done this way.

XEP-0045 doesn't support requesting history messages a-hoc.

%bmalkow Just a side note, in 7.2.0 I've implemented MAM for MUC, so there is a MAM protocol which can be used to query MUC room history.

But those messages should be sent from MUC to occupant first! If user is not in MUC room, then messages will never go to MAM.