To be fair, the list of natively supported devices is clearly provided on Hubitat's website. This forum is also a great resource to new users seeking solutions to their requirements.
HE has never been advertised as having support for every zigbee device in the marketplace. AFAIK it's only this particular tradfri device that doesn't work (which is really only a button, and there are plenty of viable alternatives that do work with HE), other ikea devices have drivers written for them or can use the built in ones.
If you must have this button working with your HE, I suggest 2 viable options are zigbee2mqtt and deconz (both routes of course do require an external raspberry pi or similar device).
Not sure native MQTT is must have for the majority of people , however @kevin (among others) has done a great job in implementing it here [Beta] MQTT beta 3d (released 5th July). I believe there other implementations as well, or you can use node red for MQTT (as I do) which opens up a ton of other possibilities.