is this still an issue?

Yes. Just tried it with Jenkins 1.634 and Scriptler 2.7. In fact, now the "ScriptlerWeb" tab in empty for me, too.

scriptlerweb is not continued anymore: JENKINS-29332 - integration will be removed with the next release

do you see any errors in the logs about github?

I've checked the Jenkins Log, no errors there.

are you able to access this file from your browser? http://updates.jenkins-ci.org/updates/org.jenkinsci.plugins.scriptler.CentralScriptJsonCatalog.json

Yes, I can wget that file from the machine / as the user running Jenkins (both URLs, the original one, and the one after your edit).

these files are downloaded via your browser, does it work there too?

You mean the file is downloaded by the client#s browser that is browsing the Jenkins' web interface? Well, yes, I can download the file also form the client's browser.

The file is not there. The only contents of the "updates" directory are:

$ ls updates/
default.json                               hudson.tasks.Ant.AntInstaller      hudson.tools.JDKInstaller
hudson.plugins.sonar.SonarRunnerInstaller  hudson.tasks.Maven.MavenInstaller

you can delete the content of the `updates` directory. Now start jenkins again, then access jenkins via browser and inspect the requests send via your browser - you should see the requests as in the screenshots I just attached to this issue.

Please let me know if you see them or not.

Please note: these files will be updated from time to time, but this can only be enforced if you delete the files from the directory.

I stopped the Jenkins service, deleted all files form the "updates" directory, and restarted the service. Although I didn't see any Scriptler files being downloaded, the "updates" directory now contains the "org.jenkinsci.plugins.scriptler.CentralScriptJsonCatalog" file, and the "GitHub" tab is populated.

It's a bit strange that I didn't see the file being downloaded, neither in Chrome or Firefox. In Chrome, which seems to be what you are using, the "Other" filter on the "Network" tab for me only shows an empty view.

strange... ...the "Doc" filter should also show it...

anyway - does mean that the GH catalog is now showing up for you?

It is, that's what I meant with

the "GitHub" tab is populated

This is whats usually is done in background (I can't recall the interval) - but I currently don't know why this was not working for you

I had the exact same problem. Stop jenkins, clear updates folder, start Jenkins fixed the problem for me too.
One note that might help analysing this problem. I did install the Scriptler plugin without reboot.. Maybe the plugin just needs a Jenkins restart to pick up the repo syncing?

Error log says:

May 03, 2020 9:42:35 PM WARNING net.sf.json.JSONObject morphPropertyValue
Can't transform property 'parameters' from java.lang.String into java.util.List. Will register a default Morpher

the issue is related to METADATA information in this plugin:
https://github.com/jenkinsci/jenkins-scripts/blob/master/scriptler/listEC2Instances.groovy

"parameters" : "",

should be and array

"parameters" : [],

Fix:
Open the file "\updates\org.jenkinsci.plugins.scriptler.CentralScriptJsonCatalog.json"
and replace the quotes ("") by the brackets ([])

save the file, now the Remote Script catalogs should populate.