Can I remove the experimental module warning from /admin/reports/status? My drupal 8 site is showing a experimental modules enabled warning under /admin/reports/status. I vaguely remember to have them enabled at a certain time, but I forgot where.

I don't use any of them, how can I remove the warning and disable the experimental modules again?

I have currently no experimental module enabled, but the warning is still displayed:
There is an experimental module that it's not shown on the screenshot given in the question: the Place Blocks module. Its .info.yml file is the following one, on Drupal 8.7.x.

name: Place Blocks
type: module
description: 'Allow administrators to place blocks from any Drupal page'
package: Core (Experimental)
version: VERSION
core: 8.x
hidden: true
dependencies:
  - drupal:block


Since it contains the hidden: true line, it isn't shown in the list of available/installed modules, in the same way modules used for the tests aren't shown.

You could ask now how it is possible to install a module that isn't shown in the list of the modules. The answer is that on Drupal 8.3.x, the module was not hidden.

name: Place Blocks
type: module
description: 'Allow administrators to place blocks from any Drupal page'
package: Core (Experimental)
version: VERSION
core: 8.x
dependencies:
  - block


Drupal 8.4.x was the first Drupal 8 branch to hide it.

As for uninstalling it, you need to edit the core/modules/block_place/block_place.info.yml file, remove the hidden: true line, or change it to hidden: false, clear the cache, visit the Extend page, and uninstall it. Be careful of not changing the line indentation for that file, as a wrong indentation can cause issues.
Since it requires to edit a module file, it could be tricky to make Drupal notice the change. I normally visit the Extend page after I edited a module file as in this case, install a core module I don't need, uninstall it, visit another page, and go back to the Extend page. At this point, I am sure Drupal caught the change I done to the module file.