(this is not the same as T342880)
We allow users to acquire temporary account usernames without actually creating the temporary account. (The username is stored in their session while logged out.) This currently happens when previewing a change in the wikitext editor (T331397 adds several more cases in other editors), and the purpose is to correctly display their signature (or username) in preview, if they add one in the edit. If the user then does not save any edits (or clears their cookies before saving), the username is never used, but it's forever reserved in the database. This entire mechanism is optional: a temporary account may be created without acquiring the name in a separate action.
Is it necessary to rate limit the acquiring of usernames, separately from limiting the creation of accounts? (discussed in T342880)
(filing as a follow-up to the discussion in https://gerrit.wikimedia.org/r/c/mediawiki/core/+/938957)
==== Why we might want rate limiting
It's somewhat far-fetched, but it may be possible to abuse this mechanism if it's not limited. Presumably there's some maximum available number of temporary usernames, but I'm not sure if it's possible to reach it. Assuming that accounts are simply numbered, increasing the numbers without creating accounts could be surprising to other users of the site, particularly since this action is not logged or otherwise auditable. It could also make it easier to grab specific numbers (see T337090).
==== Why we might not want rate limiting
It may not be worth the added complexity. In every place where we acquire a temp account username, we'll need to consider that it might fail.