Why is SPI_SLAVE kconfig option still marked "experimental"?

[palchak-google]

Describe the bug
Configuring a SPI peripheral in slave mode on a Nordic chipset causes the SPI_SLAVE kconfig option to be selected. This option is marked as "experimental". This causes a superfluous "warning: Experimental symbol SPI_SLAVE is enabled." message to be emitted during a build.

To Reproduce
Set CONFIG_SPI_SLAVE=y in any prj.conf file. Then build the project

Expected behavior
Support for SPI peripherals in slave mode has been a stable part of the SPI API for many releases. This functionality should not still be marked as "experimental".

Impact
Prevents building with "clean" output (no warnings, no errors). Misleads users into thinking code depends on potentially unstable or unreliable feature (SPI slave).

Logs and console output
Environment (please complete the following information):
Additional context