Lobel (2013:367–368) lists the following Proto-Southwest Sabahan phonological innovations that were developed from Proto-Malayo-Polynesian. (Note: PSWSAB stands for Proto-Southwest Sabahan, while PMP stands for Proto-Malayo-Polynesian.)
PMP *h > PSWSAB Ø
PMP *a > PSWSAB *ə / _# (possibly be an areal feature in Sabah or northern Borneo, since this is also found in Idaanic)
PMP *R > PSWSAB *h / (a,i,u)_(a,ə,u)
PMP *R > PSWSAB *g / ə_
PMP *-m- > ø in PSWSAB reflexes of the PMP pronoun forms *kami ‘1EXCL.NOM’, *mami ‘1EXCL.GEN’, and *kamu ‘2PL.NOM’
Reduction of most PMP consonant clusters to either singletons or prenasalized clusters
Smith (2017)
Smith (2017)[2] proposes a North Borneo group comprising the North Sarawak, Northeast Sabah, and Southwest Sabah branches.
Adelaar, K. Alexander; Himmelmann, Nikolaus, eds. (2005). The Austronesian Languages of Asia and Madagascar. Oxon, UK: Routledge.
King, Julie K.; King, John Wayne, eds. (2015). Languages of Sabah: A Survey Report. Pacific Linguistics, Series C – No. 78. Canberra: Research School of Pacific Studies, Australian National University. doi:10.15144/PL-C78.