By default the component cache lives in shared cache dir:

https://github.com/moodle/moodle/blob/main/lib/classes/component.php#L397-L399

A stack can be setup to use $CFG->alternative_component_cache but this needs to manually invalidated by some deployment scripts. It creates potential weird edge case issues when it isn't invalidated correctly.

Since MDL-82171 and related trackers landed the hash of the bootstrap is managed in localcache/bootstrap.php and I believe it should now be fully safe to also move the component cache to $CFG->localcache and correctly invalidate it as needed.

Then we can deprecate alternative_component_cache, or perhaps still continue to honor it and invalidate the cache in this other place - assuming that other place is writable.