Just to be clear, here is the text from the gotm.civfanatics.net, Code of Conduct section:
Pop-rushing
In despotism and communism it is possible to use cities purely for unit rush building. Workers can be added to such a city and then the city can then use them to rush build units. This is disallowed, so do not create these kind of cities. Pop rushing one or two regular citizens to finish a building or to build a unit is within the rules and the spirit of the game. What is against the rules is joining workers to cities for the purpose of pop rushing.
WillowBrook: it specifically mentions using Joined Workers for pop-rushing Units as being completely unacceptable, but allows a limited pop-rush of "regular citizens" to finish a building or build a unit. Unlimited Joining of Workers and then pop-rushing of anything, units or buildings, is unacceptable based on what I'm reading.
What does unlimited mean? When this rule was implemented, at least one person was found to have cities of size1 with just a specialist in them; they'd been used to pop-rush down to complete unhappiness, and with a Barracks in them, you could Join workers and use Short-rushing to generate a Veteran unit every turn and transfer the unhappiness hit to this city, so virtually no effect.
My interpretation is that units and buildings can be pop-rushed and workers can be joined to a city that pop-rushes as long as the resulting unhappiness is small enough that some citizens are still working in the city's tiles and you don't pop-rush more than the original "pre-joined" population of the city. It's impossible to say under this scenario whether "regular citizens" or Joined Workers were used to pop-rush, and they are effectively interchangeable. I don't think there's any problem with Joining workers to a city, whether it's pop-rushed earlier or not, if that city never pop-rushes again; it has already taken the unhappiness so the player is not avoiding that effect of pop-rushing, which is the intent behind this rule.
Another possible use of Workers is to Join them to a city and then Draft a defensive unit (Rifleman, Infantry, etc.), potentially to use for disbanding somewhere else. The rule doesn't mention Drafting, but since a unit is generated, I'd feel doing this violates the rule, IMO. This has a much broader potential impact, since Drafting is available under all governments, and the staff should address this specifically, especially with the focus on 100K games recently.
(edited to discuss regular citizens)