The real dealbreaker is evergreen browsers and websites updating from whatever is available on Github, making up-to-date browsers a requirement. The newest browser you can run on this is Firefox 78.15.0esr, which, funny enough, tells you that it's up to date. I also second that security by out-of-date obscurity may be a concept. (The vectors available just don't scale, if you're not a target prominent enough to make this a sensible investment. Moreover, old hardware gives you things like ECC memory, which is also immune to rowhammer attacks, as it's slow and not that tightly integrated. Having no privileged network connections to any cloud services may help, as well.)
(However, I'm writing this on a rather fresh Apple Silicon MBA, because, well, the Web…)