The only units that would have been removed or restricted from HotS Multiplayer were units from Wings of Liberty.
If they had intended to ask what units are single-player only, they should have asked that, as the question explicitly mentions multiplayer. In the original question, emphasis mine:
During development of Heart of the Swarm, there were several mentions
(e.g. the Thor, Carrier, and Mothership) of units being removed (or
restricted to 1) in HOTS multiplayer (compared to Wings of Liberty
If you wish to ask a question about what single-player campaign units are not available in multiplayer, go ahead and do so, but such a question is unlikely to solicit useful discussion, and I expect it will be summarily downvoted and/or closed. Very few campaign missions play exactly like multiplayer missions, and (I find) there is little use in comparing the two of them by units.