The tag is present in Lintian version 2.104.328. That is the most recent version we know about.
The use of _Choices in templates is deprecated. A _Choices field must be translated as a single string.
Using __Choices allows each choice to be translated separately, easing translation and is therefore recommended.
Instead of simply replacing all occurrences of "_Choices" by "__Choices", apply the method described in po-debconf(7) under "SPLITTING CHOICES LIST", to avoid breaking existing translations.
If in doubt, please ask for help on the debian-i18n mailing list.
Please refer to the po-debconf(7) manual page for details.