It seems Blizzard stealth broke the SendChatMessage function in 8.2.5:

Message: Warning: AddOn WorldBossTimers attempted to call a protected function (UNKNOWN()) which may require interaction.
Debug:
[C]: ?
[C]: SendChatMessage()
WorldBossTimers\WorldBossTimers.lua:236:
WorldBossTimers\WorldBossTimers.lua:235
WorldBossTimers\WorldBossTimers.lua:397:
WorldBossTimers\WorldBossTimers.lua:388

Emote still works.
Maybe it's worth considering using emote or a hidden addon communication channel.

The only sensible thread I could find about this:
https://us.forums.blizzard.com/en/wow/t/macroeditbox-weird-problem-after-8-2-5/319290/8

Thanks, I'll take a look at emote in 8.3. I just guess that if it works, it's considered to be a workaround that blizz forgot to disable.

Hidden addon communication only works for guild/party/raid/same-realm :(