With git+ssh deprecated, how can multiple git identities be managed?

[ParetoOptimalDev <pareto.optimal@mailfence.com>]

Hello all.

For instance with git+ssh I would have the following in my
~/.ssh/config:

```
Host paretooptimal-personal
  User git
  HostName github.com
  IdentityFile /run/agenix/pareto_optimal_personal_priv

Host paretooptimal-work
  User git
  HostName github.com
  IdentityFile /run/agenix/pareto_optimal_work_priv
```

Then I could use it like `git clone
git+ssh://paretooptimal-work/some-work/codebase.git` or
`git+ssh://paretooptimal-personal/some-personal/codebase.git`
respectively.

The problem is that since this is deprecated a tool I use in emacs called forge
doesn't want to add support for this type of git url because it's being deprecated:

https://github.com/magit/forge/issues/545

That's valid and understandable, but I feel like I have a valid use case
that calls into question the deprecation of these urls.

So my desire here is to have a solution for multiple git identities that
isn't deprecated and is supported in forge, or is a feature that can be
added using non-deprecated git functionality.

Un-deprecating git+ssh urls would be one possible solution, but if I had
to bet I'd say the case I make won't be compelling enough ;)

Either that, or some other way of accomplishing this is possible. Or
perhaps, this isn't a use case the git project thinks is worth
supporting.

I also have another case for `git+ssh` urls to create private git
repositories authenticated by ssh key. I'm fairly sure there isn't a
better solution for that.

If one needs a more concrete example of the private git repository use
case using `git+ssh`, I typically use it to create private Nix flakes.

Nix flakes: https://www.tweag.io/blog/2020-05-25-flakes/

Thank you for your time and consideration.