R-L categories correct, but text here does not match final assessment. Updated version will be published in IUCN´s Red List June or Nov 2019.
Baeospora myriadophylla is a distinct thin-fleshed small mushroom with violet lamellae. It is saprotrophic wood-inhabiting species and grows on big trunks of fir, spruce, maple or alder wood in their final decay stage. Such trunks are now a very rare sight in Europe and also threatened by forest fires in North America. At present the population seems to be stable, but it could show a decline in the next years. It is widespread over North America and Europe but rare everywhere. Altogether, it is only known from about 400 localities and the number of mature individuals is estimated to be ca. 3300, which is low, as well as the number of individuals in each subpopulation is less than 250. It is red-listed in ten European countries. The main concerns include possible decline due to habitat destruction, logging and removal of dead wood, and the habitat becoming dryer due to climate warming.
Using criterium C the result is VU C2ai, because of the small number of mature individuals, which in each subpopulation is less than 250 and altogether only 3240. The assessment is based on population size and habitat loss and habitat management changes.
Agaricus myriadophyllus Peck, Ann. Rep. Reg. Univ. St. N.Y. 25: 75 (1873) 
Baeospora myriadophylla f. cavernatilis J. Favre, Furrer-Ziogas & R. Haller Aar., Schweiz. Z. Pilzk. 29: 172 (1951)
Collybia myriadophylla (Peck) Sacc., Syll. fung. (Abellini) 5: 236 (1887)
Mycena myriadophylla (Peck) Kühner, Encyclop. Mycol. (Paris) 10: 528 (1938)
It is a distinct collybioid species with violet lamellae confined to big fir or spruce wood in final decay stage. Such trunks are now a very rare sight in Europe and therefore the species could show a decline in the next years.
It has a holarctic distribution and is present in North America (widespread, but uncommon across much of North America), Europe, from South Europe (Spain, Croatia, Romania, France) to Central (Switzerland, Liechtenstein, Austria, Germany) and North Europe (Finland, Sweden).
Based on GBIF, databases available (see below) and personal communication (Noah Siegel) 405 subpopulations are known worldwide. The number of mature individuals has been estimated at 3240 because of the likely number of current localities (estimated to be ca. 810) and the translation of the estimated total number of localities to an estimate of the total number of mature following Dahlberg & Mueller (2010). For lignicolous species few aggregated sporomata on a trunk should be counted as two mature individuals and by this a a total number of 3240 is reached. The number of known localities is 405 and the number of yet unknown localities was estimated to be twice as high.
The area of occuupancy was estimated as being 3240 km2, based on a grid cell of 4 km2 for each locality, because almost all of the basidiospores of a mature fruit body are deposited within this area (Norros et al. 2012). For taxa that have a cryptic life form (such as fungi) occurrences may be estimated by 4 km2 grid cell in which observation records are located.
Concerning its habitats, big fir or spruce trunks in final decay stage these habitats are highly endangered and declining.
Population Trend: Decreasing
Baeospora myriadophylla is a character species of species-rich mountain beech-spruce-fir forests (Galio rotundifolii-Abietenion). It also occurs in beech-fir forests (Hordelymo-Abieto Fagetum), in broadleaf forests with maple and alder (Pulmonario carpino quercenion, Lunario redivivae-Acedrion pseudoplatani, Querco-Ulmentum minoris), in Norway and North America also on Populus and in North America on Tsuga, too. It prefers fresh to moist, shady, neutral to alkaline soils, sufficiently saturated with bases and nutrients, but not to N-rich over limestone, limestone sands and marls. It grows solitary to gregarious on rotten, moss-coated trunks and stumps in the late optimal to early final decay stage, predominantly on fir and spruce, in North America also on dead alder (Alnus) and maple (Acer) on the north coast. Fruit bodies show up from late autumn to early spring, mainly from November to March.
It is threatened by habitat destruction, logging and removal of coarse dead wood. Further, it occurs in rather humid habitats during the colder winter season. The habitats may become dryer and warmer to to climate change. It is possible that increased frequency and intensity of fire may remove the coarse woody debris that this species colonizes.
To prevent decline of mycelilum loss of this species it is important to preserve larger old-growth, dead-wood-rich mixed forests, old beech-spruce-fir and spruce-fir forests in regions where it has good populations.
In North America little is known regarding this species’ distribution and particular habitat preferences. Research should also be done about dispersal capacity of this species, i.e. about spore dispersal distance and germinability.
There is no use and trade known.
GBIF.org (01 March 2019) GBIF Occurrence Download https://doi.org/10.15468/dl.wxxy4u
Krieglsteiner, G.J., Gminder, A., 2001: Die Großpilze Baden-Württembergs. Band 3. Verlag Eugen Ulmer, Stuttgart.