You can add a substring with any value at the end of the search subcommand to have sdk-manager search only for SDK versions that include the value you provided. This is referred to as [SEARCH_SUBSTR] argument by the command help.
For example, the following search command with rc string at the end will display only the versions that include rc in their names:
> nrfutil sdk-manager search rc --show-all
Version SDK Toolchain
v2.9.0-rc2 Not available Available
v2.9.0-rc1 Not available Available
v2.9.0-nRF54H20-rc1 Not available Available
v2.8.0-rc2 Not available Available
v2.8.0-rc1.1 Not available Available
v2.8.0-rc1 Not available Available