The traffic calming feature shown in the photo is a chicane (traffic_calming=chicane). The fact that plant boxes are used to create the chicane is pretty much irrelevant as far as defining the type of traffic calming feature. If you really want to get granular, you could additionally add chicane=plant_box.
please note that the definition of new tags or values have nothing to do with “devs” nor Development.
Any mapper can invent their own tags, but of course if you want support from the community (in the first place other mappers, in the second place developers of data consumers), you will have to discuss them with the community.
AFAIK the only (international) places where new tags are discussed are the tagging mailing list and the discussion pages on the wiki.
The wiki is not complete, therefore it is also interesting to look at taginfo, to see if other people had introduced values without documenting them. But in this case, I cannot find anything with “plant” as value for https://taginfo.openstreetmap.org/keys/traffic_calming#values