Some (large) networks notably allocate addressing for multiple users on a single /64. This would have a negative impact on those users.

The advent of temporary accounts has brought this from being a nice quality-of-life improvement to, IMO, a necessity if the TA system is going to continue. @kostajh I hope you don't mind if I ping you per your comment at en:WP:VPT pointing to this ticket. With /64 autoblocking, I think many of the concerns being expressed in that thread would go away quietly. There would be lower donwside to situations where admins neglect to look into an underlying IP/range when blocking a TA (@ScottishFinnishRadish' concern) and, at risk of stating the obvious, fewer TAs able to evade blocks without even trying (if they're doing one of any number of technical things that have the side-effect of bypassing a cookie-block). As a long-term goal, it might be nice to exempt ISPs known to split /64s, but in the short term I think the mild increase in collateral damage is worth it, and is comparable to what we already see with blocking schools, libraries, large mobile networks, etc. Alternatively, a middle-ground solution might be having TA autoblocks specifically affect the /64, although one would want to clearly document this divergent behavior.

I would be open to trying this. I'm not sure how we'd be able to effectively monitor for collateral damage, though.

As a long-term goal, it might be nice to exempt ISPs known to split /64s

We could potentially do this via a hardcoded list of known IPs, or by using IPoid's data.

In T377771#11491021, @kostajh wrote:

I'm not sure how we'd be able to effectively monitor for collateral damage, though.

We could include some tracking logic in the unblock process. Like, instead of serving MediaWiki:Autoblockedtext, we could create MediaWiki:Autoblockedtext-64, and ask at least the biggest Wikipedias to recommend a separate unblock template in that message, like {{unblock-auto-64}}. The way TAs are implemented already means that someone can't appeal a block through normal channels if they haven't already created a TA on that client pre-block, which artificially decreases the base rate of appeals, but I can't think of any reason that that rate would differ for /64 autoblocks versus other blocks, so we could then still do meaningful data analysis based on the rate of {{unblock-auto}} vs. {{unblock-auto-64}}.

Edited per correction below.

In T377771#11492206, @Tamzin wrote:

The way TAs are implemented already means that someone can't appeal a block through normal channels if they haven't already created a TA on that client pre-block, which artificially decreases the base rate of appeals

Logged out users should be able to use the default unblock process via user talk page even without already having a TA: T398673: Unregistered editors on blocked ranges need a way to interact on-wiki to appeal a block allows creating a TA for user talk page appeals even on blocked IPs / IP ranges (unless „prevent this user from editing their own talk page“ is enabled).