For files that still exist, this will be fixed by https://github.com/sindresorhus/refined-github/issues/2618
For deleted files, it's complex to fix it because branch/still-branch looks the same as branch/folder in the URL. Since the file is deleted, the URL is the only piece of information we have.
I looked into this. This is github's issue

#

Edit I missed the whole point of the comment 馃槅
Any time related url with a branch that has a slash that is not going to the root will fail
It's a GitHub "bug" in the sense that slashed/branch@{timestamp} URLs work for the root, but they stop working as soon as you open a file/folder. Please report it.
As for this specific bug, the URL that RGH generates is technically correct and detects the slashed/branch correctly:
https://github.com/Cog-Creators/Red-DiscordBot/blob/V3/develop@%7B2019-09-07T19:14:39Z%7D/redbot/core/events.py#L166
It's just that it doesn't work because of GitHub. I don't think this can be _fixed_ by us unless we drop the @{timestamp}
Please report it.
Submitted
Solution: don't use @{date} publicly.
We have to query the API to transform that {branch}@{date} into a commit
Easy temporary solution: use HEAD@{date} when the branch contains a slash. It's might not be correct, but it's better than a 404
Submitted
Hello yakov116,
Thanks for writing in, and we're really sorry it's taken so long to respond to you. Apologies for the delays.
I have looked into this, and unfortunately using reflog-based syntax like
Sorry about that, but please let us know if there is anything else we can do for you and we will be happy to assist!
Thanks,
Tammy
GitHub Support