With things like ChronologyProtector, we try to avoid having users make edits and then not see their own changes (often happening in redirects on the order of a few 100ms). This works well if no CDN caching applies to the URLs. When it does, the purges can suffer race conditions, such as:
a) The purge didn't make it in time (this should be rare with the udp htcp system, but you never know)
b) The purge made it but a read from a lagged slave by *another* user replaced the cache will a stale one

These are accidentally mitigated by things like SubmitAction setting a session on post, so that anons bypass CDN cache (for months). Logged in users already bypass CDN cache. However, the documented point of that was to make talk pages notices show, and it lasts for way too long just to avoid race conditions for purges. We should use an explicit cookie for that purpose. At the moment, it would be set exactly when the UseDC: cookie is set (maybe we can co-opt that one, but that seems sloppy IMO).

See also: T91820: Create HTTP verb and sticky cookie DC routing in VCL