A good senior would explain the "good reasons". A bad senior would be annoyed that the choices are questioned. As a leader I tell my "apprentices" to always question me what the reason is for doing something. Being in a leadership position is not just about your pupils progress, it's just as much about your own learning. You learn a lot when you have to explain stuff. It can be very humbling when you try to explain in-front of your team, being questioned, and then discover that you where wrong.

Often the reasons are not clear because they are not documented it, often only the dev who did it would really know.

Also, it's probably worth taking note of 'fresh eyes' because they have the advantage of hindsight which is often good. That said, there usually complex elements of incumbencies so I do think it should be some time before people have too much lean in.

This is a pretty one-sided take. I think it’s quite possible to lose perspective when working on a large codebase. It’s also possible to have small bits of technical debt accumulate without anyone feeling the total weight of it all. Old code can use whatever fad was most popular when it was written or it can carry the cruft of old dependencies or api changes. Just because something is old it doesn’t mean that it’s good

Of course. But as a newcomer, you cannot know which one it is, and as the OP said this behaviour has a lot of chances to be perceived as invalidating. I've been on both sides of this situation, and in both cases I felt that it was almost disrespectful, it's sending the message that you don't think the team is competent enough to have considered them and carefully picked trade-offs.

When I was on the receiving side (there was a newcomer to the team doing a lot of "suggestions for improvement"), I made a lot of effort to explain the context that led to a different decision, but it was very draining and rarely the good time for it (it distracts from the conversation), it was really hurting collaboration. Eventually we had a one-to-one in which I explained exactly that, and we found that it would be much better to raise these as "can you tell me why is this one like this" (rather than "improvement suggestion" that can be invalidating), and do it in one-to-one so that it's not distracting. Collaboration was much better after that

For sure, it's great to have a new pair of eyes and you certainly don't want to tell somebody to keep their challenges and suggestion for improvement for themselves. But it doesn't mean that there's no good way or bad way to do it

I typically advise new hires to keep a “dirt doc” of things they think could be improved, and have them come back to it after they are more up to speed (say a month or two in).

The insights that you get from coming at the problem with fresh eyes are invaluable and you really don’t want to waste those. But many things on that list will evaporate once you have a bit more understanding of the project.

(I still like to make “check in a fix / improvement to the dev environment documentation” the first task for a new starter though, as there’s usually a typo or update required somewhere).

Yeah, I find this a huge problem especially with junior devs (by which I mean those within the first five or so years of their career. When you have people calling themselves senior devs after six months the job title has become meaningless). Until you've been around long enough to understand how large code bases (in general, not the one you're working on!) evolve over time and why common trade offs are made you're not going to be able to grok the weaknesses of an existing code base quickly. Start off by being humble and asking why certain things have been implemented in the way they have, there is usually a reason. Sometimes it's even a good one!