Hi Matthieu,
I wish the request was more popular, because to me it is also painfully obvious!
At the moment, Logseq is more of a write-once, read never, notataking system.
Other than org-mode I am not sure which system has proper tag hierarchies, but I am sure many of them do, given that this is such a basic idea. A couple people in the threads gave up and moved on to other systems that do, unfortunately I don’t remember which.
Alex0 has been in touch with the developers and his proposal for a basic tree search seems to have gained traction, he mentioned that it would be one of the next items they’d work on. This thread has more info, it also has a diagram how it would all work.
It is not a big project, all we need is a tree search command, the UI probably can already render the output. Ultimately we’ll need some tools to efficiently organize tags, but worst case this can be done with an external script.