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) proposes a North Borneo group comprising the North Sarawak, Northeast Sabah, and Southwest Sabah branches.