Not what I am talking about.

I am talking about using the same key pair for two different things. E.g. GitHub and GitLab

What attack vector do you think this would mitigate?

You'd need several prerequisites for that to actually increase security, i.e. not using an ssh-agent (required password for every key prompt) and encrypting them at rest

Otherwise you'll leak all your keys as soon as any attack vector is utilized such as hostile host siphoning from the ssh-agent forwarding or filesystem access.

I want to be able to revoke individual keys. If I want to invalidate my key for any reason, I don‘t want to have to change it for everything.

but why do you want to do this.

there is no security benefit from revoking individual keys unless they've been compromised - however, the likelihood of only leaking a single key is extremely unlikely.

There are very few attack vectors how you can compromise a private/public key pair and they all basically boil down to local access. This is not a PreSharedKey situation like a password, where both parties effectively share a single string for authentication. The private key never leaves the authenticating machine, as you're only sending a signature over which will be validated against the public key. So, how are you going to compromise a single key that splitting them increases your security?

you're either completely compromised and somebody has filesystem access or you've forwarded your SSH-Agent to a compromised host. When its the former, you'll have to have the private-key encrypted so they're unable to use them (encrypted at rest) and when its the later, you cannot have your keys added to the agent, making the forwarding redundant in the first place.

Using different keys for different services is overkill most of the time.

Generally you need to have one key per host that you use (or per any storage location). You can use separate keys for separate services is if you for eg. privacy reasons don't want to associate the same identity with both services, but that's a personal choice, not something that improves security.

I am. My ssh key is my gpg auth key. It's stored on my yubikey requiring a touch each time I use it.

I use the same gpg key for talking to anyone, so why wouldn't I do the same with my ssh key? I suppose I could try to build a workflow around host-specific keys somehow derived from my auth key… but I'd need some reason to do so.