I have read the answers about deorbiting second/upper stage. But since the lastest launch of Long March 5B raised concern about its uncontrolled reentry (yeah, too big). Unlike SpaceX's Falcon, other rockets are not designed for reuse, do people still care about intentionally deorbiting the first stage, and how?
I have read the answers about deorbiting second/upper stage
do people still care about intentionally deorbiting the first stage
It doesn't matter whether it is a first stage, second stage, third stage, fourth stage, kick stage, space tug, satellite dispenser, satellite, space station, tank, or something else entirely.
It is not a good idea to have anything hit the ground uncontrolled.
In fact, it also doesn't matter whether it reaches orbit or not. Debris falling uncontrollably is bad, period. Regardless of whether it came from a first or second stage, and regardless of whether it reached orbit or not.
That's why there are Flight Termination Systems, for example, or why there are Launch Corridors.
The reason why we typically do not hear much about specifically deorbiting specifically the first stage is simply the fact that normally, first stages usually do not reach orbit. So, for almost all launch systems, the question is moot.
The Chang Zheng 5B is a special case.
But launch providers most certainly do care about where our first stages end up. For example, the Space Shuttle deliberately did not achieve orbit using the Space Shuttle Main Engines to make sure that it could drop the External Tank on a sub-orbital trajectory, then used the Orbital Maneuvering System thrusters to boost the Orbiter into orbit. It would have been simpler and more efficient to skip the OMS burn and use the SSMEs to go all the way to orbit, but that would have left the ET in an uncontrolled orbit.
Note: technically, this is not 100% answering your question, because in this case, we deliberately prevented a big hunk of metal from reaching orbit in the first place, rather than intentionally deorbiting it after reaching orbit. I would still argue that the intention behind the decision is the same, though.
As a rule-of-thumb, it is considered to be irresponsible to let anything above 10t reenter uncontrolled. This is just a rule-of-thumb, of course, since e.g. a 10t modular space station, and a 10t titanium cannonball would behave very differently because of their different ballistic coefficients: the modular space station with its oddly-shaped modules, solar panels, and radiators would probably be ripped into multiple smaller pieces which are more likely to completely burn up in the atmosphere.
Note that some debris from the last Chang Zheng 5B mission's first stage did fall on a village in Côte d'Ivoire in Africa. Miraculously, nobody was hurt. But the ground track also passed over densely populated areas of the United States, for example, so if the stage had reentered just a bit earlier or later, the results may very well have been very different.
I am not just talking about the potential loss of life and property damage caused by the immediate event, but also the potential political fallout from Chinese military hardware raining hell on the US during the very strained relationship of the Trump Presidency.
The premise of this question is flawed First stages do not reach orbit, they are suborbital and fly on ballistic trajectories. So the question has never arisen at least up until now. It is theoretically possible to build a single stage to orbit vehicle and the Lockheed X33 experimental vehicle almost made it but was cancelled prior to being tested. https://www.thespacereview.com/article/3815/1
That said there is usually concern about where sub orbital first stages land both from the point of view of reuse as well as safety. However the Chinese seem to be particularly willing to accept risks far in excess of what would be acceptable in the West. Presumably due to the fact that it is an authoritarian state. There are many instances of rocket debris falling near human habitation in china.