Yes, I agree that if the legal texts you quoted :
are indeed the actual rules in Bavaria that define on which paths are /are not allowed to ride your bike then it is so hard to verify that the lesser evil would be something like “bicycle=unknown” with a note explaining why that value was chosen.
However -IANAB (I am not a Bavarian 
)- the link you posted also contains texts that -together with articles below from cycling associations- might suggest that access for bicycle=* may have become more clear and verifiable with those changes instead of less clear. Perhaps something along the lines of :
“access is allowed for cycling, unless there is a barrier or sign that says that is is not, and the landowner is free to place those, some conditions in hindsight” :
Which may roughly be seen in OSM as bicycle=permissive on highway=path
I am interested in your view on this.
For those interested, see the sources below.
Vollzug des Bayerischen Naturschutzgesetzes
Blocking by the landowner or other authorized persons, Art. 27 Para. 3, Art. 33 BayNatSchG
2.6.1 General
1.The right of access may not be exercised if the property owner or other authorized person has prohibited access to his or her property – whether permissible or not (Art. 27 Para. 3 Sentence 2 BayNatSchG).2. The prerequisite is that the prohibition has been imposed by means of clear blocks applicable to the general public.
[…]
9. If the property owner or other authorized person blocks his property with signs, these must refer to a legal reason that justifies a restriction of the right of access (Art. 27 Para. 3 Clause 3 BayNatSchG), for example “forestry work”, […]
10. If such a reason is not given, such blocking signs for those seeking recreation are irrelevant.
11.This applies, for example, to signs with the inscription “Private property – Trespassing prohibited”.
And here is the actual Bayerisches Naturschutzgesetz – BayNatSchG itself to which the above mentioned “Volzug” refers. (The Volzug if I understand correctly is a service instruction to the authorities on how the law -here: BayNatSchG- is to be implemented? )
The cycling association DIMB (who -although from non-neutral perspective of course - have a extensive legal database), seem to interpret the notions on suitable ways in BayNatSchG here, and here as when a way would be deemed not suitable for bicycles, it is not bicycle=no, but that the property owners are not liable for damages or obliged to maintain paths for cycling.
And with the new Volzug it is argued that the cycling bans have become more clear with signs and presumably more frequent (see also the second half of the link in the quote " wave of signs…"):
When the new administrative regulation for the Bavarian Nature Conservation Act came into force in December 2020, cycling associations such as the DIMB feared a wave of signs prohibiting mountain bikers . Because the new regulation is intended to create an “objective determination of drivability”. In practice, this means that authorities and landowners can more easily enforce road closures. If MTB trails are closed, this must be indicated with appropriate signs.
The crux: Not only the lower and higher nature conservation authorities are allowed to put up prohibition signs, the landowners themselves are also authorized.