Is it intended that standard tools keys not overwrittable when running in deprecationSupportMode?
Contrary to the new view.ViewtoolContext, The view.context.ChainedContext does not allow standard tools keys to be overwritten.
If I remember correctly, tools keys were not overwrittable in 1.x. So it may be regarded as a normal behaviour not to allow this in deprecationSupportMode, but it is not, since tools keys have changed! Better allow tools keys to be overwrittable, since this hardly breaks compatibility (maybe for broken templates).