Cannot address whether the rule changed or not, but here is the MLS Next sub rule:
https://bsbproduction.s3.amazonaws.com/portals/116/docs/mls%20next%20rules%20and%20regulations%202020-2021%20-%20final%20-%209.8.20.pdf
Page 35
b. Substitutions:
Substitutions are allowed at any stoppage. Games shall follow the Technical Framework provided for
at the beginning of each MLS NEXT Season. The maximum limits for substitutions are:
For U-14, U-15, U-16, U-17, and U-19 MLS NEXT Games there will be unlimited substitutions over three (3) moments in the game, plus half time with no re-entry after exiting the match.
For U-13 MLS NEXT Games, there will be unlimited substitutions over three (3) moments per half, plus half time, with no re-entry per half.
Head Injury Substitution: All substitutions relating to head injury, including whether to charge such substitution, shall be handled in compliance with the PDev Concussion Protocol.