tl;dr Don’t use links like
github.com/owner/repo/blob/master/file.ext#L13 because that branch will change! Use
github.com/owner/repo/blob/COMMIT_SHA/file.ext#L13 and be future-proof.
Are you creating a link to a specific line of code, or a particular file in a code repository? That file may change or move in the future which means your link will eventually be worthless: the file you’re referring to can be edited which means your link will highlight the wrong line of code, or the whole file itself could move or be renamed which means your link will be a 404 error!
You’re in luck: it’s very easy to avoid this problem. Find the most recent commit in that repository, and link to the file/line using the SHA from that commit. The commit itself (should!) never change, so even if your line of code gets deleted in the future, your link will still work and will still highlight the correct code.
The way I do this on GitHub is to visit the project’s root page (e.g. github.com/alxndr/lyriki). Just above the list of files on that page, there’s a row which shows the most recent commit (e.g. at time of writing
@alxndr bump patch version, Latest commit 82ff021 5 days ago). Both the commit message and the SHA there are links to that commit’s page (e.g. github.com/alxndr/lyriki/commit/82ff0210…); visit that page and then click the "Browse files" button on the right-hand side of the commit message header. You’ll be taken to a page which looks like the project’s root page (e.g. github.com/alxndr/lyriki/tree/82ff0210…), but now you’re looking at the repo at a specific point in time instead of whatever is most recent; notice that the URL includes "tree/COMMIT_SHA/…" (if you’re at a directory) or "blob/COMMIT_SHA/…" (if you’re at a file).
Now you can click through to find your target file (or use the
t shortcut to fuzzy-find it), and notice that the URL for the file also includes "blob/COMMIT_SHA/…". That URL will continue to work and show this file, even if the file is renamed, moved, or deleted in the repository’s future. Magic!
(This does not, however, protect against the repository itself being deleted; if you’re worried about that, clone the repo and link to your fork. It also doesn’t protect against GitHub going away; if you’re worried about that, I dunno what to tell you.)