What road hubs models or manufacturers use replaceable cartridge/sealed bearings?
Maybe approach this the other way around. Identify the wheels you like (one or more model) and as part of your purchase decision, see how easy it is to source replacement bearings. 

If there are many and varied places to get the bearings from, not limited to cycling retailers, then you know getting replacements will likely be straightforward. You’ll have done the best you can for now. 

6903 RS is a bearing size I’ve seen often enough. 

I haven’t got to the heart of the question of course, I also would still be interested if other people know whether there is anything like an industry standard right now. 

Update: one resource is a poster produced by Elvedes of the aftermarket bearings they produce.
https://elvedes.nl/wp-content/uploads/2018/07/Elvedes-Poster-v4_web.compressed.pdf
Maybe not exhaustive or guaranteed for the future, but could prove helpful.