Update from 06 September:
The classic school of etiquette is now exerting its influence on the Shadowlands. Or to put it another way: With patch 9.1.5, you can no longer spit at targeted players in retail WoW either. Only if you enter /spit in the chat without a targeted player, the familiar message “You spit on the ground” will appear. With a player in the target, nothing happens.

Source: Icy Veins

Original news from July 31:
Surprisingly, visitors to the public test server for the second phase of WoW: Burning Crusade Classic noticed that it is no longer possible to spit at other players. That is, if you type /spit in chat without targeting a player, the familiar message “You’re spitting on the ground.” will appear. However, as soon as you target a player and then trigger the emote, nothing happens.

Since this change landed on the PTR without accompanying hotfix documentation or explanatory developer commentary, players are now speculating on the reason for the change.

Perhaps it’s because critics of the Deluxe Edition have been banning users from the game every chance they get. Reawakened Phase Hunter spit on. Maybe they also want to limit the players who at every opportunity NPC… Field Marshal Afrasiabi spit at, which has not yet been adjusted on the Classic servers, as it is happened on the Retail servers with all the Afrasiabi allusions.

Maybe after all the bad news and reports that have come to the public around misogyny, suicide, sexual harassment, alcohol escapades, and more, they just want to start slowly tolerating no more space for bad behavior of this kind in general. Alternatively, of course, this could just be a bug.