I've seen a few people report non-use of render files on export, allegedly unique to new FCP versions. However they provided insufficient details to independently verify.
From my limited testing thus far on 10.5.1, it seems to work about like prior versions regarding render cache optimization on export. However the whole render file issue is complex with many variables. There could be additional cases where it's gotten worse, or maybe it hasn't and we just recently noticed.
In Jan 2020 I had a very heated debate with Pro Apps escalation support about this, who after discussing with product support management and FCP engineering, decided it was not a bug and they do not guarantee any specific cache file behavior regarding render or export performance.
They said that has no bearing on the matter. I then kept them on the phone and walked them through a replication procedure, and had them tell me the # of seconds to execute each case. They could not deny render cache files improved export performance.
They then said that any observed render or export performance optimization (or lack of it) due to render cache files is not documented and they will not address it.
I followed this up with a detailed email to them summarizing that discussion and including a simple replication scenario.
Since then I've been meaning to revisit this, but it would take several days of testing to re-verify the current behavior across many scenarios. It seems quite complex and involves certain combinations of certain effects (whether built-in or 3rd party). My gut feel is it may involve FxPlug 3 architecture and how that interacts with the render system. That is currently being replaced with FxPlug 4 but it will take a while. I'm not confident they would act on any additional enhancement request, but maybe I'm just being negative.