Here is some information about the top level of the Wikidata ontology, as of 10 August 2023, along with some comments about their instances and subclasses and what I think should be done with them. As well, I think that the top level classes should have good Wikidata usage instructions that describe better what the classes are for. I have put Wikidata usage instructions on fourth-order class (Q24027474) and fifth-order class (Q24027515) as examples.
Summary of the top-level ontology classes with some commments (<- is number of instances, <= subclasses)
entity
<-
<=
instance of
subclass of
comments
first-order class (Q104086571)
44
0
second-order class (Q24017414)
fixed-order class (Q23959932)
all instances look OK, no interwiki links
second-order class (Q24017414)
423
4
third-order class (Q24017465)
fixed-order class (Q23959932) , metaclass (Q19478619)
many instances questionable, no interwiki links
third-order class (Q24017465)
22
0
fourth-order class (Q24027474)
fixed-order class (Q23959932) , metaclass (Q19478619)
many instances questionable, no interwiki links
fourth-order class (Q24027474)
1
0
fifth-order class (Q24027515)
fixed-order class (Q23959932) , metaclass (Q19478619)
unused, no interwiki links
fifth-order class (Q24027515)
1
0
fixed order class of higher order (Q24027526)
fixed-order class (Q23959932) , metaclass (Q19478619)
unused, no interwiki links
fixed order class of higher order (Q24027526)
1
0
variable-order class (Q23958852)
fixed-order class (Q23959932) , metaclass (Q19478619)
unused, no interwiki links
fixed-order class (Q23959932)
0
6
variable-order class (Q23958852)
class (Q23960977)
unused, no interwiki links
variable-order class (Q23958852)
15
0
variable-order class (Q23958852)
class (Q23960977)
many instances questionable, no interwiki links
metaclass (Q19478619)
80
23
variable-order class (Q23958852) , concept (Q151885)
class (Q23960977)
linked to wikipedia pages for metaclass in the semantic web, some instances questionable, many subclasses questionable, move most instances and subclasses to one of the fixed-order classes
metaclass (Q19361238)
476
9
metaclass (Q19478619) , class (Q21522864)
no interwiki links, many instances questionable, many subclasses questionable, merge with second-order class?
entity (Q35120)
21
45
variable-order class (Q23958852)
class of all entities, linked to many wikipedia pages
other high-level classes
class (Q5127848)
3
8
metaclass (Q19478619)
abstract entity (Q7184903)
linked to wikipedia pages for class in logic and philosophy, move incoming links to class (Q16889133)
class (Q16889133)
253
237
metaclass (Q19478619) , variable-order class (Q23958852)
linked to English wikipedia page for class in knowledge representation, most instances are first-order classes, many subclasses questionable
abstract entity (Q7184903)
17
63
variable-order class (Q23958852)
object (Q488383)
object (Q488383)
708
88
variable-order class (Q23958852)
entity (Q35120)
Q104093226
1
0
class (Q16889133)
delete?
concept (Q151885)
2668
91
variable-order class (Q23958852)
abstract entity (Q7184903)