Why there is not a long-term on v7?
if i upgraded my router from 6.48.6 long term to 7.3.1, Is there any problem that effect user-manger and hotspot

True long-term v7 will come as 7.12.x or something. Before that there will simply be too much development to truly stabilize something with decent working feature set.

version 7.1.5 is now considered, but it is not yet what it was in V6

Why there is not a long-term on v7?
if i upgraded my router from 6.48.6 long term to 7.3.1, Is there any problem that effect user-manger and hotspot

It will be ready when its ready.
What you can do is to test the 7.3.1 on a equal router as you have and see if all function works as expected, then you can do the upgrade.

If there are noe reason (new function you need or error that can not be solved with upgrade), you do not need to upgrade.

Well, v7.4 just came out with a bunch of bug fixes.