/plain doesn't work with replies or edits

[tulir]

When trying to reply with /plain, the reply is just ignored. When trying to edit with /plain, the /plain is taken literally and the message is parsed as markdown

[grinapo]

same with

/plain - hello
- what
- nothing

[aaronraimist]

Since the reply part has been fixed maybe this should be closed in favor of #16531