If you click on https://www.wikidata[.]org/w/index.php?title=Special%3AWhatLinksHere&target=Property%3AP31&namespace=1&invert=1 it'll take more than thirty seconds to load and at this rate can be simply turned into a DDoS attack vector.
It's already being used by users (and that's how I found it, from slow queries logs):
https://logstash.wikimedia.org/app/discover#/doc/logstash-*/logstash-mediawiki-2021.12.14?id=DoYeuH0B-N5J53KJweTL
The query:
SELECT page_id,page_namespace,page_title,rd_from,rd_fragment,page_is_redirect FROM (SELECT pl_from,rd_from,rd_fragment FROM `pagelinks` LEFT JOIN `redirect` ON ((rd_from = pl_from) AND rd_title = 'P31' AND rd_namespace = 120 AND (rd_interwiki = '' OR rd_interwiki IS NULL)) WHERE pl_namespace = 120 AND pl_title = 'P31' AND pl_from_namespace IN (0,2,3,4,5,6,7,8,9,10,11,12,13,14,15,120,121,122,123,146,147,640,641,828,829,1198,1199,2300,2301,2302,2303,2600) ORDER BY pl_from ASC LIMIT 102 ) `temp_backlink_range` JOIN `page` ON ((pl_from = page_id)) ORDER BY page_id ASC LIMIT 51
Explain:
*************************** 1. row ***************************
id: 1
select_type: PRIMARY
table: <derived2>
type: ALL
possible_keys: NULL
key: NULL
key_len: NULL
ref: NULL
rows: 102
Extra: Using filesort
*************************** 2. row ***************************
id: 1
select_type: PRIMARY
table: page
type: eq_ref
possible_keys: PRIMARY
key: PRIMARY
key_len: 4
ref: temp_backlink_range.pl_from
rows: 1
Extra:
*************************** 3. row ***************************
id: 2
select_type: DERIVED
table: pagelinks
type: range
possible_keys: pl_backlinks_namespace,pl_namespace
key: pl_backlinks_namespace
key_len: 265
ref: NULL
rows: 205355406
Extra: Using where; Using index; Using filesort
*************************** 4. row ***************************
id: 2
select_type: DERIVED
table: redirect
type: eq_ref
possible_keys: PRIMARY,rd_ns_title
key: PRIMARY
key_len: 4
ref: wikidatawiki.pagelinks.pl_from
rows: 1
Extra: Using where
4 rows in set (0.002 sec)
ERROR: No query specifiedIt tries to scan 200M rows! What makes it even more dangerous is that Special:Whatlinkshere is not among special pages I'm planning to put a cap on (T297708)

