As described in https://bitbucket.org/blog/sunsetting-mercurial-support-in-bitbucket bitbucket will delete hg repositories in less than one year from now.
For an example, I linked to the thg repo and one of my own in How to set up Diffuse as Mercurial's 3-way merge tool and save from the middle pane into target
Given the timeframe, I can probably manually add comments or edit answers and questions I did, which are not that many. However, what should be done more generally? (Also, realistically my replacement links to my partner's server are not ready for long-term use either.)
If the original owner of a repo is still around then, I believe, they can re-register their to be deleted hg repos as git repos with bitbucket. (I will probably do that too.) However, at least the links with commit IDs in them will break then, unless bitbucket provides more extensive switchover support (none of which seems to emerge).