It will be supported until 2025, as documented here.
That’s also the end-of-life date for Ignition Citadel, the first Ignition LTS.
should I start with v11, or use Gazebo ignition?
It depends on your use-case and the features you need. If Gazebo 11 fits your needs, there’s no reason not to use it. On the other hand, if you need some features which are available in Ignition, but not on Gazebo 11, then you can use Ignition Citadel, for example. Here’s a feature comparison list. Ignition is rapidly getting new features, but for now, it still misses a lot of the tools, APIs and usability that Gazebo-classic offers.