Well I’m from Denmark with 3 months required by law (if you’ve been employeed more than 6 months. 1 month if its <6 months.), so I completely agree. Sometimes it just usually distracts from the conversion if you mention danish basic rights to americans :-)

3 months notice, while retaining access to company networks and campus?

I support "keeping them on the books" for purposes of visa and health insurance, for example, but there are legitimate security reasons to have people cut off quickly when they've been laid off or fired.

I don't think anyone advocating the 3 months thinks you need to keep them doing something. You just need to keep providing the benefits (salary, health if in the US, visa, etc.) for 3 months. Even in the EU you don't have to let someone come in to the office during that time.

I suppose if you had really, really, really good meals provided, it would be reasonable to argue about whether that was a benefit or there for people in the office.

A company can give them 3 months "paid time off" while also revoking their access to tools/systems,.

I've never been in the situation, but from what I gather then both things happen. Sometimes your are given notice (and pay) but access revoked. But its also my understanding that in many cases, maybe even most, the employee works in the 3 months. The employer may even be a bit more lax with time off so the employee can do interviews with other companies. I know in most cases the notice period is used for handing over projects/work to another employee at the company, so stuff doesn't just get dropped. I get the security aspect, but from my experience its just not something an employee usually does. Maybe some other Danes can pitch in with their experiences.

I think its b/c getting fired is bad ofc, but not bad like in the US. I mean we have universal basic income, free healthcare etc. So I just don't think people feel pressured in a way that drives them to exploit access even after a notice, or get mad at the company in the same way. I'm just speculating. Again, there are companies and positions in companies where its a no brainer that access is revoked for security reasons.

Remaining vacation days should cover the better part of one month alone. If you don't want them to come in for whatever reason, you can send them on paid leave.

Most companies stop having "legitimate security reasons" when the alternative is to pay people for doing nothing. Turns out the fact that actual sabotage and theft of trade secrets is illegal (and often represents a breach of the employment contract, which can incur hefty fines) is a sufficient deterrent for most industries.