then the most recent IP address by which the temporary account is blocked for the same duration as the temporary account.

So, autoblock already does this for 24 hours. Which makes it useless in its own way.

Your solution is also useless, because IPs may be used for a much different period by one person than the temporary account (the practice of blocking which is either indefinitely or 90 days: oops, you just blocked an IP indefinitely).

I do wonder as well if there may be better solutions than what I suggested. Maybe some sort of IP hashing so that one can know if two temporary accounts share the same IP without actually sharing the same IP.

Or maybe the behavior should be that it places two separate blocks with different block settings. So an admin can block a temporary account indefinitely yet block an IP address for a fixed period like one year. This is something that may be worth working on for when temporary accounts gets deployed off of Wikimedia (such as Fandom, since I don't think Fandom staff would be comfortable letting anyone outside of a few global volunteer groups having access to IP addresses for accounts, temporary or registered).

If the issue is about autoblocks affecting registered users (I don’t think I've heard about a single case in the past couple of months, per perhaps that’s different on enwiki?), the solution might be introducing "temp account autoblocks" instead of adding new block options to manually block an unknown IP address.

Btw I don’t understand why the admin can’t just enable their IP access via Special:Preferences if they want to check whether different temp accounts belong to the same person? Accessing the IP would also allow them to place an IP block without any autoblock issues if they're concerned about that.

In T411068#11409972, @Johannnes89 wrote:

If the issue is about autoblocks affecting registered users (I don’t think I've heard about a single case in the past couple of months, per perhaps that’s different on enwiki?), the solution might be introducing "temp account autoblocks" instead of adding new block options to manually block an unknown IP address.

Btw I don’t understand why the admin can’t just enable their IP access via Special:Preferences if they want to check whether different temp accounts belong to the same person?

A wiki or wiki farm might want to not configure access to temporary account IP addresses for administrators. I named Fandom as a possible example of this.

If someone does this on LocalSettings.php on a wiki:

$wgGroupPermissions['sysop']['checkuser-temporary-account'] = false;
$wgGroupPermissions['sysop']['ipinfo'] = false;
$wgGroupPermissions['sysop']['ipinfo-view-full'] = false;

Administrators should still be able to block the underlying IP addresses of temporary accounts without them knowing what those IP addresses are.

So, autoblock already does this for 24 hours. Which makes it useless in its own way.

See also: T43479: [Spam/vandalism] Raise $wgAutoblockExpiry noticeably. The current issue is to measure which autoblock expiry is proper.