In migrating categorylinks to abstract schema I accidentally changed datatype of cl_timestamp from timestamp to binary(14). I was about to fix it but I realized this column is not used anywhere. Even its documentation says it's not used at the moment:
This isn't really used at present. Provided for an optional sorting method by approximate addition time.
The problem is that this documentation was added in 2005 (commit 5fdbef206f35adeb11ee0b97894f8fdb5c451a0e) the field probably exited even before that and not used until now and it's not even not fixed along other timestamp fields (T42626: Standardise type of timestamp database fields (MySQL)).
The idea of cl_timestamp is delicious but keeping an empty field (plus its index) on such a large table (in commons it's 200GB) for more than 15 years because it might get used one day(TM) seems a bit excessive to me. We can add it back again when resources to implement such feature is provided.