This is an important announcement for all plugin developers (and pre-release users) - from the next pre-release (v2.2.4), all deprecated plugin calls are converted to errors. It means that any plugin still using such API will no longer work.
Most deprecation notices have been around for about six months and have been printing warnings in the console since then, so most should be fixed by now. However if you notice any plugin no longer working please let me know and notify the plugin developer too.
One plugin I've noticed is the "Outline" one, which still uses the old registerSetting API.
The reason for turning old deprecation notices into errors is that new developers might copy and paste code from deprecated plugins, thus potentially making deprecated code more and more widespread, and making it harder to remove that old code later on. Some of that old code causes crashes and issues that are hard to debug so it should be removed.
Simply run the plugin while the console is open. In there search for "deprecation notice" or "no longer supported" and that should list the deprecation issues along with instructions on how to fix them.
In that case someone else will have to fork and publish again the plugin (provided it's open source). There's no easy way around this unfortunately. All these deprecation notices happened while the plugin system was still actively under development so if a plugin was built at that time and didn't fix the deprecation issues there's not much to do other than forking.