Here are my unstructured notes from playing with Special:IPContributions:

• There is no simple way to switch from "viewing contributions of a temporary account" to "viewing contributions from all temporary accounts from the same IP". When I'm in the page history context, this is simple: I need to reveal the IP, click the IP and I see all the temporary accounts from there. However, the same cannot be done from Special:Contributions/TEMP_ACCOUNT (I need to open a random diff, reveal IP and click it, which is not intuitive). Should we add Reveal IP functionality to Special:Contributions as well?
• When I query via an IP range, it is not possible to easily see what IP from that range belongs to what temporary account. When deciding to make a range block, I need to know how many distinct addresses edit from that range, and as of now, I'd need to go diff-by-diff and make notes, which is very tedious. Since we already accessed the IP address of the temp accounts to display their contributions, would it be reasonable to simply display the IP there? Or should we add reveal IP there, and rely on bulk reveal (this would mean an additional click though)?
• When someone changes the target to a non-IP while at Special:IPContributions, they are presented with an error ("Not a valid IP address"). While this is factually correct, since the new special page looks exactly like Special:Contributions (and the only distinction is the URI), this is likely to be confusing. I recommend to automatically redirect to Special:Contributions instead.
• Similarly to my previous point, some power users use shortcuts such as typing Special:Contribs/XXX to the searchbox. When this is done with IPs, then historical contributions are displayed. I find it challenging to identify the "right" behavior is (and maybe the current behavior is correct). Over time, I presume functionaries would be more interested in temporary than historical contributions, but the answer "in the day of deployment" might be different. I recommend thinking about the expectations (fellow stewards/functionaries should also feel free to express their opinions).
• Navigation between the two modes of Special:Contributions is hard to find. There are links in the tools menu, but those are easily lost among the dozens of different tools. Would it make sense to give the mode more affordance? For example, can we use something similar to "visual/source" editing switching? This will likely make the previous two points somehow less significant, as it would be more clear what mode is the current one, and why the behaviour is different.
• AFAIK, Special:IPContributions is a time-limited view (contributions older than 90 days will not show). This is not clear from the user interface, and users might get easily confused about why are records disappearing. We should probably add an explanatory notice (and possibly even explicitly mention the cutoff date, if that is possible).

I'll take another look and make another comment if needed.

@Tchanders @Madalina @KColeman-WMF, please let me know if you have any questions about this feedback.

Next step is to process the feedback from T366780#9867816 and make new tasks as necessary.

@kostajh Looking at testwiki, I believe that all the feedback has been addressed in the improvements that were made to Special:Contributions and Special:IPContributions.