Absolute truths I unlearned as junior developer

"Loads of companies and startups have little or no tests" which should scare you, or at least it would scare me if I joined a team.

You can definitely over test, but how can you possibly know what you built works (or still works when you change it for the 50th time) if you have no tests? There's a trade-off with testing. Early in the dev cycle, not testing can make you go super fast (supposedly, this hasn't been my personal experience but in general it seems to be true for teams). But you'll plateau quickly and then at iteration 1+n you'll just come to a screeching halt because you introduce bugs or the new engineer isn't confident that they didn't break downstream things and needs to manually test everything. Testing early will cause you to go slower earlier (again, not my personal experience but seems to be generally true of teams) but you'll be significantly faster at iteration 1+n.

I usually summarize this as: testing early will on average make your development faster over the life of your codebase. Knowing this you can make trade-offs. Not testing early is probably better called prototyping. It's OK to prototype in production if you need to. But know what you're getting yourself into.

I call this the intrinsic quality of the code. As you say, it isn't that it does not matter at all, just that it is over indexed by many of us.

Admittedly, my first days as a junior programmer were before some of you were born, but I'm thinking of a particular format here...

Learned as junior: If you report an OS bug or some other deep problem, seniors will not believe you and assume you're making excuses for your own bugs and lack of understanding.

Understood as senior: If a junior programmer tells me they found a system-level bug, I won't believe them and will tell them to go figure out what's wrong with their code.

Learned as junior: Legacy code is hard to read.

Understood as senior: Legacy code that I wrote myself is hard to read.

Learned as junior: Technical skills matter most.

Understood as senior: Communication skills matter most.

Learned as junior: New tech solves old problems.

Understood as senior: New tech creates new problems.

There are some programs that are not designed to last. It doesn't make sense to put in time in that regard.

A workflow using the code is a itself a type of test. If something breaks, the users will tell you. A lot of code ends up in that space because it was never asked for as an actual project, but now a team relies on it but no developer can be officially assigned to it.