Should reform ‘siddurim’ be put in shaimos or burned?
The siddurim should be put into shaimos, and not burned. See the following post https://dinonline.org/2017/11/20/disposal-of-messianic-prayerbook/ regarding Messianic prayer books. Messianic prayer books contain outright clear heresy and apikorsus, therefore they should be burned, however the siddurim of the Reform, although I have never read one, it is unclear that they do contain outright heresy, therefore we can’t say that they are considered a “sefer shkosvo min”. (Although the person who put out the siddur might be considered an apikorus, but regarding the siddur it wouldn’t be considered “kosvo min”). See Igros Moshe Y:D 1-172.
R’ Y. Feinhandler shlit”a Author of Ginzei Hakodesh.