Looked into this, the base links are:

• Talk
• Contribs
• Block
• Send Email

Without doing additional prep work:

• Talk pages will always be blue, as there won't be an external check to see if the talk page exists
• Contribs won't support redContribsWhenNoEdits (UserRights sets this to false so it's no difference in this use case

Block and Send Email do checks on the local wiki which I would prefer not to check on the external wiki as this adds an API call. If this is the case, then should these links always be shown? Or never shown?

Thanks @STran

In T413216#11480994, @STran wrote:

Without doing additional prep work:

• Talk pages will always be blue, as there won't be an external check to see if the talk page exists
• Contribs won't support redContribsWhenNoEdits (UserRights sets this to false so it's no difference in this use case

I think this is fine - seems better to have a wrongly-coloured link than no link.

Block and Send Email do checks on the local wiki which I would prefer not to check on the external wiki as this adds an API call. If this is the case, then should these links always be shown? Or never shown?

We could remove these, perhaps. @Johannnes89 Do you find these links particularly useful when editing the groups of a user at an external wiki?

In T413216#11491225, @Tchanders wrote:

Thanks @STran

In T413216#11480994, @STran wrote:

Without doing additional prep work:

• Talk pages will always be blue, as there won't be an external check to see if the talk page exists
• Contribs won't support redContribsWhenNoEdits (UserRights sets this to false so it's no difference in this use case

I think this is fine - seems better to have a wrongly-coloured link than no link.

Agree.

Block and Send Email do checks on the local wiki which I would prefer not to check on the external wiki as this adds an API call. If this is the case, then should these links always be shown? Or never shown?

We could remove these, perhaps. @Johannnes89 Do you find these links particularly useful when editing the groups of a user at an external wiki?

I've never used block and email links via Special:UserRights. meta:Special:UserRights already doesn't show those links, instead there are talk, contribs, membership in global groups – different to en:Special:UserRights and other wikis (talk, contribs, block, send email).

Change #1224118 had a related patch set uploaded (by STran; author: STran):

[mediawiki/core@master] Support user links for external users on Special:UserRights

https://gerrit.wikimedia.org/r/1224118