is it planned, that CI will be extensible with custom actions?
Yes, planned for a future release. Still some work to do in that area.
What this ever implemented. Searching I only find items discussing doing custom actions in FinalBuilder and Automise.
It’s implemented in our v2 code base (on dot net core 5.0) - however dot net core’s lack of appdomains, coupled with how it handles dependencies is proving to be a challenge. We use the api’s internally (even in v1) so we know that part works, of course for stuff we add we can control the dependencies - but we don’t want to get to a point where installing a plugin that uses a different version of a nuget package will break things. Of course it all also needs to be documented.
Hopefully we will have that issue resolved before we get to the release stage of v2 later this year - I don’t have an exact timeframe, but work on it is progressing.