TYPO3 Core

Did you try TypoScript overrides? https://docs.typo3.org/typo3cms/drafts/code.tritum.de/TYPO3.CMS/Form_Documentation/Concepts/Index.html#typoscript-overrides
It's not exactly what you want, but sounds useful and I guess you could use constants there.

Yes already tried that. For me the approach with constants within the YAML file feels cleaner.
Nevertheless I tried some finisher overrides (for example the mail subject) and it does not seem to work in all cases...

The TS overrides are ok but constructs like

plugin.tx_form {
  settings {
    formDefinitionOverrides {
      SomeForm {
        renderables {
          0 {
            renderables {
              4 {
                properties {
                  privacyPageUid = {$plugin.someExt.general.privacyPagePid}
                }
              }
            }
          }
        }
      }
    }
  }
}

are not really nice since you have to hard code the indexes in the array which fail when something gets added in between in the configuration file. It would be a lot better to be able to use TS constants in the YML file.

The use of variants is really nice and useful but with constants you have imho some advantages.

• You can centralize the things that need to be changed when moving to an other production stage. So simply changing or adding condition to one file will have effects to the complete installation.

• When email addresses change, you will have one place to check and update.

• It is possible to change constants within the page tree in a sub template (will break the two other advantages).

Or are all these things possible with #84133?

Indeed Christian Ludwig is right.

For example we use constants as the one single entry point for configuration of the whole TYPO3 installation depending on locale, context and other conditions. We configure everything from one constants file sitting in our main extension. (every custom/sysext/third party extension including its plugins, modules etc.)

Variants look nice but I don't get why we need to duplicate all the functionality already available with constants. In addition it spreads the configuration all over our setup and adds another configuration point.

Variants do blow up the YAML file even further when it could be as easy as just reading the value from a constant.

Thanks to #84203 you can now use placeholders in form setup files. With this, you can set up a form element definition using e.g. environment variables.

Notice that placeholders (and imports, see #84204) are currently still not supported in form definition files. That's why you need to use form setup files and form element definitions therein.

Does this help already?