So,
I can answer my own question:
The call “/rest/addons” does not (anymore) return all bindings that are installed!
To really get ALL installed bindings per rest, each addon service must be retrieved…
This can be done as follows:
- retrieve all services via rest: “/rest/addons/services”:
[
{
"id": "jar",
"name": "JAR-File add-on service",
"addonTypes": [
{
"id": "automation",
"label": "Automation"
},
{
"id": "binding",
"label": "Bindings"
},
{
"id": "misc",
"label": "Misc"
},
{
"id": "persistence",
"label": "Persistence"
},
{
"id": "transformation",
"label": "Transformations"
},
{
"id": "ui",
"label": "User Interfaces"
},
{
"id": "voice",
"label": "Voice"
}
]
},
{
"id": "karaf",
"name": "openHAB Distribution",
"addonTypes": [
{
"id": "automation",
"label": "Automation"
},
{
"id": "binding",
"label": "Bindings"
},
{
"id": "misc",
"label": "Misc"
},
{
"id": "persistence",
"label": "Persistence"
},
{
"id": "transformation",
"label": "Transformations"
},
{
"id": "ui",
"label": "User Interfaces"
},
{
"id": "voice",
"label": "Voice"
}
]
},
{
"id": "marketplace",
"name": "Community Marketplace",
"addonTypes": [
{
"id": "automation",
"label": "Automation"
},
{
"id": "binding",
"label": "Bindings"
},
{
"id": "misc",
"label": "Misc"
},
{
"id": "persistence",
"label": "Persistence"
},
{
"id": "transformation",
"label": "Transformations"
},
{
"id": "ui",
"label": "User Interfaces"
},
{
"id": "voice",
"label": "Voice"
}
]
},
{
"id": "json",
"name": "Json 3rd Party Add-on Service",
"addonTypes": [
{
"id": "automation",
"label": "Automation"
},
{
"id": "binding",
"label": "Bindings"
},
{
"id": "misc",
"label": "Misc"
},
{
"id": "persistence",
"label": "Persistence"
},
{
"id": "transformation",
"label": "Transformations"
},
{
"id": "ui",
"label": "User Interfaces"
},
{
"id": "voice",
"label": "Voice"
}
]
}
]
- iterate over the id’s of the result of the services and thus query the bindings: “/rest/addons/services?serviceId=ServiceID” (e.g. “/rest/addons/services?serviceId=jar”)
This way you get all installed bindings again 
And this is also how the MainUI does it…
Nevertheless, I am sure that up to and including 4.3.2 the call “/rest/addons/services” was sufficient to get all installed bindings, including those from the addon folder…