Another idea is expire temporary account that is inactive for some shorter time (e.g. 7 days or 30 days). But we need some way to store how temporary account is recently used (maybe just store it in cache?)

In T359653#9615869, @Bugreporter wrote:

Another idea is expire temporary account that is inactive for some shorter time (e.g. 7 days or 30 days). But we need some way to store how temporary account is recently used (maybe just store it in cache?)

Thanks for this idea--I've filed it as T359789: Expire inactive temp accounts after 7 days, and we should add some more detail to it.

much shorter duration, like 7 days, might also be desirable

This would also mean that temporary accounts can be contacted via their talk page, or be mentioned on other talk pages for that short period, right? 7 days is a quite narrow timeframe to review temporary account's edits, and for temporary account user to also return and receive the feedback. For this reason it migh be better to keep a temporary account (inactive or not) for some longer period, maybe at least a month or so.

Also, I don't know how exactly the temporary accounts feature changes user blocks, but I assume too short duration also might make blocks less efficient than desired (at least concerning blocked users who don't know about session cookies).

but I assume too short duration also might make blocks less efficient than desired (at least concerning blocked users who don't know about session cookies).

For LTA, a potentially useful feature is to also block every IP used by the account (or even /24 range thereof) in one click. Normal block of temporary account can be bypassed by removing the cookie, and autoblock will expire after one day until T27305: Add a way to extend autoblock to longer than 1 day/T43479: [Spam/vandalism] Raise $wgAutoblockExpiry noticeably.

Let's prioritize this task. We can reassess after testwiki deploy if we see evidence that we need to keep them for longer.

Change #1047930 had a related patch set uploaded (by Kosta Harlan; author: Kosta Harlan):

[mediawiki/core@master] [temp accounts] Set expiration to 90 days

https://gerrit.wikimedia.org/r/1047930

Change #1047930 merged by jenkins-bot:

[mediawiki/core@master] [temp accounts] Set expiration to 90 days

https://gerrit.wikimedia.org/r/1047930