민수기 36:6 해설: Kitzur Ba'al HaTurim, Malbim, Or HaChaim & Rashbam

In the Gemara, it is mentioned that this was practiced only in that specific generation.

This is the command that the Lord has given concerning the daughters of Tzelafchad: This command specifically relates to the issue of the tribe's portion, and therefore, for their benefit, they shall be wives means that even if the husband is not from their own family, but from the family of their father's tribe, if he is from the tribe of Menasheh, even if he is not from their immediate family but from another family within the tribe, they shall be wives.

זה הדבר, "this is the thing, etc." This expression is explained in Baba Batra 120 as follows: "the daughters of Tzelofchod were permitted free choice of husbands, including husbands who were from different tribes, as it says in our verse לטוב בעיניהם תהיינה לנשים, 'let them be married to whom they think best;'" the question then arises how we can can reconcile this with the words "but let them be married to members of the tribe of their father?" The Talmud answers that these words are not a condition but merely sound advice by the Torah to these girls. Thus far the Talmud. When the Torah said זה הדבר these words were necessary because what preceded them was the Torah's acknowledgment that legally speaking the tribes of Joseph had a sound claim. The Torah therefore had to go on record that the justice of the claim of the tribes of Joseph extended to other daughters inheriting land through their fathers which might become lost to their tribe if such daughters married someone out of their tribe and the Jubilee regulation might alienate such land to the original tribe forever. However, in this single instance of the daughters of Tzelofchod, the general rule did not apply and these daughters were free to be married to men of their choosing even men outside their tribes. The word זה is restrictive and makes an exception of the daughters of Tzelofchod. This is also alluded to in the unnecessary word לאמור in this verse. G'd went out of His way to pay great honour to these girls.