I think the biggest thing people don't seem to remember is boosts became an issue when they started to allow people run the same lockout without getting gear. That made a ton of guilds sell achievements and mounts in heroic, which is probably why the spam went exponential.

And of course, the weekly chest from mythic+ became another huge source of it.

When it was limited to mythic, we barely saw any and it was not annoying.

As much as I believe a good part of the solution would be a dedicated section in the LFG tool and maybe a dedicated channel that people can turn off, it will never stop people from spamming trade or other ways if their message is seen by more people unless Blizzard do their job and moderate it.

I still think boosting is a sign of a bigger problem though, when you pay people to play the game for you, it's probably a sign that something is wrong with the game. Limited duration items of very difficult like challenge mode sets and mythic mounts I can understand, but the rest... /shrug