I hope this package isn't dead because I love it and it is broken!
If you download the bookdown demo, open up _output.yml, and add msmbstyle::msmb_html_book in there (with any set of options), then rendering the book will produce the error:
Error in load_config()[[type]] : missing subscript
I believe this is an issue introduced with R 4.3, since that error message is new as of 4.3.
This error is a result of calling source_link() without specifying a type argument in the msmb_build_chapter function.
I created a PR that fixes the problem. I'm just adding this Issue here in case this package is abandoned, in which case any passerby can install my version using remotes::install_github('NickCH-K/msmbstyle') to fix the problem.