Individual smartapps are no longer needed for most use cases since the new V3 app provides multiple features for creating rules which are much more sophisticated than the older generations, as well as easier to use.
These include:
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The official “smart lighting“ feature. These are the automations which are eligible to run locally.
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The official “SmartThings home monitor“ feature. These automations can trigger security notifications.
https://support.smartthings.com/hc/en-us/articles/205380154-SmartThings-Home-Monitor
- Custom automations. This is what you create when you use the + in the new V3 to “add an automation.” These allow for multiple triggers in a single rule, timers, delays, different types of notifications, etc. At the time of this writing, these automations run in the smartthings cloud.
These first three are all that most smartthings customers will need to set up the rules for their system. 
In addition, there are two more options for people with a strong technical background or who have a particular very complex need.
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the official “Rules API.” Still in development, this feature is a full-fledged rules engine with variables and other programming features.
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Webcore. This is a highly sophisticated community – created rules engine which has been around for a couple of years. It’s essentially a scripting language for the smartthings groovy platform. Very powerful. shortly after its introduction, Most developers stopped writing individual smartapps as webcore just provided them With a much more effective way to do the same thing. Individual rules in Webcore rules are called “pistons,” and community members do share them with each other, usually through the webcore forum, which is very active. See the FAQ (The topic title is a clickable link).
So the short answer is that we don’t need a lot of smartapps anymore. The rules engines options listed above are much more powerful, easier to use, and quicker to set up. 
This process began in 2016 and is now close to complete except for the continuing work being done on the rules API. If you check the following page from a couple of years ago you can see where many individual smartapps were being retired and the functionality moved to the official features. (‘Routines” are no longer used in the new V3 app, but have been replaced by the built-in rules engine for automations.)