Stealth support for proprietary url schemes for imagery

[SimonPoole]

It seems as if JOSM supports ESRI imagery servers within the "WMS" type of source by supporting {wkid} placeholders (and other proprietary attributes).

Nothing against the principle of supporting such servers, however I believe it should be in a form that makes it clear that this is non-standard and proprietary, and not not confuse users in to thinking that this is part of a standard. With other words, I would suggest that such servers get their own type value (which will make validating such entries easier too).

[taylor.smock]

I don't think I'd classify {wkid} as a proprietary placeholder. It is {proj} without the prefix (usually EPSG). It may have been added specifically to make Esri imagery servers easier to add, but I doubt it was intended to apply specifically to those servers. TBH, I think most users adding information to the imagery list just hardcode the projection code. Just look for bboxSR and imageSR in https://josm.openstreetmap.de/maps . Without {wkid}.

I don't know what other "proprietary attributes" you could be talking about.