Vanish notices are essentially useless traffic, especially if the accounts aren't going to be used again per user request.

Lock actions are not secret, so this is certainly not an inappropriate information disclosure problem; no issues with it being turned off if it is useless noise though.

In T372022#10050428, @Xaosflux wrote:

Lock actions are not secret, so this is certainly not an inappropriate information disclosure problem; no issues with it being turned off if it is useless noise though.

I don't think Vanishes are. At least I couldn't find anything in meta's Special:Log

They are. The locking just happens before the rename.

E.g.
lock: https://meta.wikimedia.org/w/index.php?title=Special:Log&logid=56924943
and then rename: https://meta.wikimedia.org/w/index.php?title=Special:Log&logid=56924944

c.f. T372024 about if this account should be +system, +bot