Adding metadata from JSON file as text to QGIS print layout

You could use json_to_map() to turn your json into a dictionary and then get the value of 'creation_time' by using map_get() to search for that key. Here an example, (I made a full json out of your example, copying a random one from the web):

map_get(json_to_map('
{
   "crs":{
      "type":"name",
      "properties":{
         "name":"[EPSG:code]"
      }
   },
   "license":"[license info]",
   "mapname":"[name of the map]",
   "map_long_name":"[despription of the map]",
   "map_short_name":"[short name]",
   "map_abstract":"[abstract of the map]",
   "creation_time":"[timestamp]",
   "type":"FeatureCollection",
   "features":[
      {
         "type":"Feature",
         "properties":{
            "name":"codecentric AG",
            "address":"Hochstr. 11",
            "marker-color":"#008800",
            "marker-symbol":"commercial"
         },
         "geometry":{
            "type":"Point",
            "coordinates":[
               7.0069,
               51.1623
            ]
         }
      }
   ]
}
'),'creation_time')

This example will return '[timestamp]'. Depending on how you access this data, you need to hand it over as string to this function.

---

I could not find a possibility to load data from external urls via expressions. So I created this custom function:

from qgis.core import *
from qgis.gui import *
import urllib.request
import urllib
import json

@qgsfunction(args='auto', group='Custom')
def jsonfromurl(url, feature, parent):
    header = {"accept":"application/json"}
    request = urllib.request.Request(url)
    response = urllib.request.urlopen(request)
    response_data = response.read()
    encoding = response.info().get_content_charset('utf-8')
    data = json.loads(response_data.decode(encoding))
    #txt = json.dumps(data) #uncomment this line to receive data as string. also change next line to: return txt
    return data

It basically accesses the json content from a given url. My test url is http://api.plos.org/search?q=title:DNA. In case your url does not return a json by default, you may need adjust the second line to request = urllib.request.Request(url,header). In case you want to receive your json as string, uncomment the second last line and change the last one to return txt.

Example usage: map_get(jsonfromurl('http://api.plos.org/search?q=title:DNA'),'response'), so in your case maybe map_get(jsonfromurl('yoururls'),'creation_time').