When it comes to typeface design and protection of said designs, there are a few angles to consider:
- the law
- professional ethics
- personal ethics
We'll forgo personal ethics for now as that is highly debatable depending on one's personal opinion.
As for the law...it's...vague and messy, at best. And highly dependent on your region. Wikipedia has a decent high level overview of the topic. In the US, the general rule is that the actual letter forms' shape is not copyrightable design. However, the code (ie, the font itself) can be copyrighted as software code. In addition, the name of the typeface can be copyrighted as well.
So, legally, in the US, I could draw my own version of Frutiger that looks almost exactly alike, name it something else, and sell it and it'd be perfectly legally.
Is it professionally ethical? I'd argue, no. However, there's arguments that are valid to say 'yes' as well. So it really depends.
Where it gets even muddier is when you look at things like 'typeface revival' vs. 'typeface rip-off'. Some typefaces are 'revivals' in that they are based on an existing design. They are marketed this way. Some typefaces are seen as just blatant rip-offs. What's the difference? Is there a difference? Hard to say generically as it usually comes down to context and intent.
And then there's the issue of 'so similar as to be suspicious'. Adobe has Myriad. Is that a 'revival'? A 'new and improved' version? Or is that just a plain rip-off? Depending on who you ask, you'll get a different answer.
It should also be noted that historically, ripping off type designs was how the industry worked. There was a time when all type was physical...made out of wood or metal (lead). A foundry (which was literally a foundry in that they poured cast metal out of forms) would design a typeface and then sell it through their catalog. If that typeface became popular, a competing company would want to offer their version, which would be a redrawing of the same typeface with a new name.
In summary: Is Frutiger in the public domain? The answer is: It depends. :)