As far as I know, Lightroom 6 requires you to be online only at the time you apply the serial number; then after that you can leave the computer offline forever and Lightroom 6 will work.
I don't think it was even possible with LR 5 or maybe even LR 4. At some point you have to connect the computer to the internet so the Adobe program can check the license status, usually when you first install the program. This only needs to be done once IIRC. Supposedly it is the same with LR 6 stand alone. But don't quote me on that
is there any way how to install Lightroom to a permanently offline computer? I know it was possible for the previous version even it wasn't very easy.
If you have an LR6 (perpetual) license, you may have to be connected when installing. The procedure should be the same as with previous versions. If you subscribed to LR CC, you have to connect while using LR (or while Adobe CC Desktop is active) at least once each month (or something like that).
However, LR6/CC has a bug that causes trouble and may eventually crash the program if you are not always connected when using LR. LR tries to connect to the Adobe servers about every 20 seconds. This has been discussed in other threads. So if you want to work disconnected, wait for version 6.1.