Feel free to add, I guess I am ok with uk|au and some other one’s, your call which you want added, just edit it in here: https://github.com/discourse/discourse/blob/master/config/site_settings.yml#L540
Note we can also make per locale defaults here, so Chinese get .cn for free, etc.
I think .org also needs to be included by default here. .org is one of the original TLDs, and still used a lot today. We even use it! www.discourse.org is no longer linked.
If we want to limit it to 3, I’d much rather see .org than .gov.
Feel free to add org, I forgot it
Here’s the top domains by observed requests, grouping by public suffix according to Cisco Popularity List :
com, net, org, (googleapis.com), io, co, tv, (amazon s3), (amazon ELB), ru, cn, goog, us, uk, me, de, (co.uk), fr, fi
That’s 15, not counting second-level effectiveTLDs.
I am not a huge fan of adding tv, io or goog … ru does have a tiny bit of risk cause it can be mistyped in English and magic linked.
I am ok to add uk and co, some of the other domains as well in lang specific locals.
Keep in mind https://test.io is magic linked, this is just about people that magically remember to type in test.io cause … reasons?
crates.io/crates/embedded-hal is a plausible URL to write out while typing.
I’d rather not have every foo.thing auto-linked. Mistyping ‘. Foo’ as ‘.Foo’ is such an easy one to make. It would be terrible to have a conversation flow distracted by too many links.
If people want links, they should learn how to do it properly. Hyperlinking is an essential part of Internet technologies, and I certainly don’t want to encourage people ignoring it. See EU Copyright reform Article 11 (to criminalize hyperlinks). Instead, Internet users should become aware of why such things get hyperlinked, and what it entails. Having com|gov|net|org is fine (although I wonder why gov and not mil since they are of same generation, and US-specific – I’d say remove the gov 
) but managing the growing ICANN TLD mess should be avoided IMO, or part of a plugin.
Why not whitelist every TLD that is not a noun?
I personally don’t see why whitelist anything really, it is just so rare for people to type in domains, just like that, vs copying a link. Also discourse.org loses on https now which is not great, cause now my link is slower.
I personally don’t see that either but since you already opted to build a whitelisting mechanism I chose to continue on that idea
Let’s build a whitelist of sites that are verified to work with https:// so that can be the default protocol for them.
No, just kidding. But at least http will eventually go away while we will be seeing more and more TLD’s that look like a noun.
Maybe this is also an opportunity to delinkify example.com and mydomain.com?
This is such a worm can 
I would say that if you don’t want particular domains linked, just disable the feature via the site setting
OK per Restrict domain name autolinking so M.Sc and B.Sc are not auto linked this is done.