> Oh. That's too bad. Why is that? I mean why drop something that is working

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> and might be of need for at least some people?

Wow, you sure like to make presumptions.
It was dropped because it was unmaintained and unmaintainable.  It did

not fit into the system, and it did not work.
> Anyway. As it doesn't matter why - is there any documentation about how to

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> compile a kernel module for PCI ISDN-cards?

We would not know.  We don't use it.
> Which is the latest kernel with *official* ISDN-support, and which ISDN card

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> would be recommended for *BSD?

I dunno, ten years ago?  If it worked at all, that is.
> If ISDN/i4b is still maintained on FreeBSD, it should not be to much of an

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> effort to get it running on OpenBSD as well, right?

You'd be surprised.  Thing is, you won't get help here.