Why ubuntu software centre, which is the app store for ubuntu never show about application updates which is one of the basic app store functionality?
Why can't I update applications without upgrading the whole OS?
I know that's why they brought snap apps and snap store. But it has its limitations. Instead of that, isn't that good that bringing a stable release update app feature? What they haven't come up with yet? We have to check manually for new updates and the software centre doesn't. How can we know when new updates will come?
They do for some select applications, such as Firefox. But otherwise it is by design.
There is nothing stopping you to install new "stable" releases of other packages in an LTS version of Ubuntu, if you have the knowledge to do so. But they will not be published in the official repositories for your version.
You either have to:
All of these methods require some skill and could compromise your system or change the look and usage of the software.
If you want it the way you propose, you should consider using a rolling release distribution, like Arch or Manjaro