Hoi!
If, for example, de.wiki admins would forbid the usage of de-1901 in de.wiki as a non-official variant, will it be ok?
Yes, if the community is okay with it. Maybe I could be personally against the decision, but I don't vote in de.wiki (and I would not want to, I spend more than enough time in wmf "as is").
Why in the free encyclopedia one must not write in the language variant, let it be non-official now, which is in use, in which books and newspapers were and are printed now?
The variant in object is NOT official in any state. As a linguistic entity it is not alternative to official Byelorussian (no ISO code). No matter how many books and papers you print for us it simply does not exist. We do not have ANY rule "for Byelorussian" (or for any other single language). We treat Byelorussian just like anybody else.
I'm positive that whatever rule we will choose there will always be cases in which the blanket is short. Yet by having a single public rule (instead of making "ad hoc" votes) we at least avoid single arbitrary decisions. There will be no change for this rule because changing it would require making a "Byelorussian exception".
The wmf cannot be a place in which exceptions are made, because once you have exceptions you have special people who get special services (or less services than normal people do) based on the fact that they have personal positive or negative relations in the wmf. We want to maintain a situation in which all linguistic entities are equal and decisions are made based on factors that judges cannot control.
The day in which a single exception will be accepted (no matter which one) I will immediately resign from LangCom. All my work is spent to avoid having people kissing the asses of bureaucrats and admin communities by needing to request individual permissions based on "ad hoc" decisions; I'll better take my buttocks out of the Committee before becoming a feudal power myself.
To get rid of feudal powers we need clear public policies that anyone can read and apply long before asking anyone. The process is far from being over, yet we are working on it and hopefully the amount of arbitrary moves required to allow a project will progressively collapse to zero. Not without cases like yours, but that's life, sorry. There is no such rule like a rule that has "everybody happy".
The main problem is the smaller project be.wiki doesn't want to unite with the bigger be-x-old.wiki.
Smaller or bigger doesn't mean a thing to us. Be also aware that AFAIK there is still an open proceeding that at least theoretically might end up in closing be-x-old definitely. IMHO you should get at least ready to migrate be-x-old content into a united version, in case that happens. No content and none of the existing privileges (admins and bureaucrats) would be migrated by the wmf, if and when such a decision is made.
Public quarrels won't help, you'll simply end up in making the situation between the two communities worse. No matter your orthography/accent diplomacy works much better if you have real humans meet in front of a real beer (or whatever else), instead of periodically exchanging accuses here.
Berto 'd Sera Personagi dl'ann 2006 per l'arvista american-a Time (tanme tuti vojaotri) http://www.time.com/time/magazine/article/0,9171,1569514,00.html