The current concept of sub-projects is questionable, not well documented, decreases performance and there are certainly bugs in it.
I remember that @vboctor had the feature on the deprecating / obsoleting list, but I commented that it can't be removed because of existing installations.
I assume that @vboctor introduced the option to avoid dealing with sub-project issues on MantisHub.

I assume that @vboctor introduced the option to avoid dealing with sub-project issues on MantisHub.

OK, that makes sense, if this option is to avoid the creation of subprojects by the administrators.
In that case, the documentation should be updated to make it clear.

The current concept of sub-projects is questionable, not well documented, decreases performance and there are certainly bugs in it.
I remember that @vboctor had the feature on the deprecating / obsoleting list, but I commented that it can't be removed because of existing installations.

i already commented on that topic too, that there is a severe functionality regression if removing it.

Turning off sub-projects is the logical goal of the config option. At one point, it may be just hiding sub-projects and disabling their creation, but in the future, we could add checks to remove extra/expensive queries that support sub-projects, etc. Hence, I wouldn't document that it just disables creation of sub-projects, but this can evolve over time. I would hope that we have this feature disable by default for future versions.

At one point, it may be just hiding sub-projects and disabling their creation, but in the future, we could add checks to remove extra/expensive queries that support sub-projects, etc. Hence, I wouldn't document that it just disables creation of sub-projects, but this can evolve over time

Then document that!
Whatever it may do in the future, explain what it does now.