This refers to D2ROffline and D2RModding. The former allowed users to play without a network connection, which is required by default in Resurrected, while the latter unlocked all characters restricted in the alpha version.
Both mods violated Blizzard’s modification policies. The creators received letters stating the need to cease their “unlawful” activities.
This does not mean that mods will not be supported in the full version of the game. Blizzard simply made it clear that it does not intend to tolerate creations that pose a potential threat to its security policy. A company representative commented on the situation:
“We recognize how great the modding community’s contribution is to the longevity of Diablo II, and we appreciate their enthusiasm for the game. (…) We will do our best to support mods for Diablo II: Resurrected. But some of them are unusual and pose a threat to our product. And because we have always prioritized security, this type of software is unacceptable.”