This is a pretty good article on the origin of the spliceosome from self-splicing introns:
Vosseberg J, Snel B. Domestication of self-splicing introns during eukaryogenesis: the rise of the complex spliceosomal machinery. Biol Direct. 2017 Dec 1;12(1):30. DOI:10.1186/s13062-017-0201-6
There are many interesting references given in that article also, for example on spliceosomal diversity. There are simple protozoan parasites known with as few as 27 introns in their entire genomes, and highly simplified spliceosomes with only ~35 proteins. It is pretty clear that the spliceosome has in fact evolved to become more complex in some lineages, having grown from a conserved core of about 60 proteins in the last eukaryotic common ancestor, to about 100 proteins in yeast, and over 200 proteins in mammals. While in other lineages like protozoan parasites it has shrunk in terms of the number of proteins and RNAs.