The documentation for the planCacheSize parameter could be improved in a few ways.

First, it may be worth clarifying that this parameter only affects the plan cache for the slot-based execution (SBE) engine. There is a separate plan cache for the classic execution engine which does not obey this parameter. As we ship more and more features in (SBE), we expect that the SBE plan cache will become the primary plan cache.

Second but perhaps more important, the docs state that this was new in version 5.1. That's stale and presumably the result of the fact that we originally planned to ship SBE and the SBE plan cache in an earlier version but ultimately chose to delay the release. We ended up shipping SBE for some limited $group/$lookup scenarios in 6.0 and did not release the SBE plan cache. In 6.2 we shipped some additional $lookup nested loop join plans in SBE. In 6.3, we did a much more broad release of SBE for find commands and also enabled the SBE plan cache. In 7.0, we plan to release a bit more so that I would expect most find commands to execute using SBE in 7.0.

The implications for the docs are that the SBE plan cache was first enabled in 6.3. Since the planCacheSize parameter is tied specifically to the SBE plan cache as explained above, this means that the planCacheSize parameter should be documented as new in 6.3 rather than new in 5.1.