"Full stack" is a bit of a pretentious nitpick on my part, I actually hesitated putting it in there but couldn't resist. I'm an embedded developer who does a good amount of low level/bare metal work, so when I hear about engineering positions requiring being able to do basic Linux sysadmin + write some Python/PHP/JS/HTML/CSS/... referred to as "full stack" I always want to arrogantly point out that this is actually quite a long way away from the "full" stack. I guess it's just an other symptom of web technologies at large eating a huge portion of software development.
In my understanding it has more to do with a person's ability to learn, or lack of fear from learning, if you wish.
Of course I have met my share of "full stack" devs (PHP + JS), but that's the same as with "senior" devs... Having a job title and being one is not the same thing.
I've met quite a few who are happy specializing and essentially restricting themselves to a couple of technologies. When they call themselves "full stack", I get a bit annoyed, since I feel it should mean more than that.
So yeah, if you give me a month, I'm sure I can become productive in any given stack, and given a year, I'll be mostly indistinguishable from a specialist.
I still provision my own Linux (VPS) servers, administer MariaDB on them and string it all together with PHP and sprinkle in non-trivial Javascrpt for UI and graphing. My company is profitable, customers are happy, and my software feeds 2 families well.
My methods are antiquated, sure, but if me telling people I'm a full stack developer is laughable/wrong, then I'm truly lost.