More info, copy-pasting from my Discord community:
The primary functionality lost for GSE is the ability to use clickable macros, which facilitated gameplay on controllers and handheld devices. Blizzard did not specifically target GSE, but rather made changes to combat botting.
Blizzard’s bot detection improvements likely impacted how programs like GSE interact with controllers. This is because bot programs often add human-like inaccuracies, making them harder to detect. GSE’s functionality might have been exploited to create more undetectable bots. Therefore, Blizzard’s changes, intended to improve bot detection, inadvertently affected GSE’s controller compatibility.
While these changes disrupt legitimate users, they are part of necessary measures to maintain game integrity by preventing botting.