I work with SP 2019 on Prem. One of the strangest problems to notice.
I have enabled SharePoint Server Publishing and SharePoint Server publishing infrastructure. If you're editing a classic page, you'll see below that there are no web parts available for the content / script editor
See configuration for the web app level below,
I've also disabled and re-enabled the SP Server publishing feature, but still no luck.
When I create a brand new site collection under the same web application, I see all the classic web parts, script / content editor all, no problems. So I think the site collection is a specific problem.
If anyone has ever experienced this, please shed some light and thank you in advance for your help.