The system allows to synonymize tags where the resulting tag does not have a wiki, but the source one had. How in that case I can find the wiki of the source tag without breaking the synonymized pair?
The orphaned wiki page doesn't contain that wiki.
Here are a couple SEDE queries which return:
I used Stack Overflow as an example here - but in SEDE one can switch to any site on the network.
Keep in mind that SEDE is only updated once a week. (So you won't see the most recent synonyms there - you'll have to wait for the next update if you need to see one of those.)
And I will add a collection of some other tag-related queries which might sometimes be interesting.
The OP suggested in comments that one could obtain the old tag-info simply by going to tags/{old-tag-name}. However, this only works sometimes. In the example I mentioned above, the link https://stackoverflow.com/tags/agile-processes/info redirects to https://stackoverflow.com/tags/agile/info.
However, https://stackoverflow.com/tags/azure-devops-server/info does not redirect to https://stackoverflow.com/tags/tfs/info - despite being a synonym.
One conjecture mentioned in the comments was that the redirect is only added if the tags are merged - but I did not find any sources to confirm this.
