If we're talking serious vulnerabilities (e.g. can allow remote code execution) for a project used by a large number of people, then absolutely. It can be weeks between initial report and a fix being pushed depending on the vulnerability, and even longer for people to update. It's a pretty good niche: intercept emails to maintainers for important projects and either sell the vulnerabilities or use them yourself in that small window. It'll net you a pretty steady stream of 0days if you can pull it off and you choose the right target. Encrypting communication makes that strategy a lot more difficult, and more invasive.

It just seems to me that the window is small (and you have to write a weaponised exploit in that time too), and the only people likely to be able to reliably intercept the email are nation-state actors who do bulk email slurping, and they're probably sitting on a pile of zerodays anyway. But that's just gut feeling so it's likely wrong.

I'll admit it's not a likely scenario, it's just good trade craft.

The window isn't as small as you'd think. The hacker would have all the malware written except for the infiltration point. And a skilled team, who is familiar with the software you are targeting, can go from a whitepaper to something actionable in a couple weeks. Less if they don't sleep much. Even better, a lot of these vulnerability reports come with POCs that can be pretty easily adapted to your needs.

As for who could pull this off, you don't necessarily need nation-state resources. In fact, bulk email slurping probably wouldn't help, since STARTTLS is pretty ubiquitous. Since you're focusing on a single target, your best bet is compromising the SMTP server. If that's someone's personal server, that's well within the abilities of a moderately skilled group. If it's managed email (say gmail) it gets much harder, but maybe they get lucky and figure out your password, either from good guessing/brute force or from an account leak.

Of course, a group could get access to those vulnerability reports by hacking the laptop of someone with the keys, but at least using PGP lowers the attack surface.

This is probably a niche use; all the potential RCE vulnerabilities I've found in projects with large numbers of users have involved projects with no formal vulnerability reporting process at all, where just finding a private way of reporting the issue was a pain.

Using pgp is good for +30 cryptolluminati cool points. It also reduces your kibitz surface, which is useful for keeping the guardian from writing incorrect hit pieces about how you don't care about security.

Other than that, not really.