The question is whether a separate switch, specifically for enabling/disabling keywords, makes sense.
I do not think so. It is one thing to want to force one's code to adhere to a particular strict standard (e.g., EP or SP), and it is quite another, to turn on some features of one "standard", while turning off others. IMHO, if one is not coding to the strict requirements of EP or SP (or perhaps, when ready, ANSI OP), then one might as well just accept GPC's strength in supporting a wide variety of dialects, with the understanding that this might require (in the case of pre-existing code) some changes to the names of variables or data types that clash with keywords in the supported dialects.
Or, like FPC did, implement per "standard" a strict mode and a loose mode.
The strict for keeping legacy apps running, the loose mode for interoperability.
But it will require extra work, but on the other hand, maybe it is worth it, because with increasing compiler complexity, more of these problems will arise, and maybe it is wise to setup an infrastructure to deal with it now already.