We’re using these plugins.
https://github.com/discourse/discourse-bbcode
Previously <span> would allow some thread post modifications. The previous admin added this to the whitelist. A user identified a possible clickjack exploit which I reported and the resulting update killed the ability to use <span>. If we whitelist again, it might work but will likely be wiped out in any new updates. This will not happen (or so I’ve been told) with a proper plugin.