Hoping that @paul.buerkner or anyone else might be able to weigh in on whether it is (still?) possible to include monotonic spline terms in models fit with brms. I saw a thread from Jan '19 indicating that it is workable, though possibly involving some modifications to the scam pacakge. However, when I run the minimal working example posted on that thread (here) after modifying the scam source code as suggested, the resulting smooths are decidedly not monotonic. Of course, alternative suggestions for incorporating monotonic smooth of continuous covariates would be also welcome. Thanks!
# minimal example
library(brms)
library(scam) # N.B. this is compiled from source with modifications per referenced post
b = brm(time ~ s(age, bs = "mpi"), data = kidney)
marginal_smooths(b, spaghetti = T, nsamples = 10)
# system info
version
_
platform x86_64-apple-darwin15.6.0
arch x86_64
os darwin15.6.0
system x86_64, darwin15.6.0
status
major 3
minor 6.2
year 2019
month 12
day 12
svn rev 77560
language R
version.string R version 3.6.2 (2019-12-12)
nickname Dark and Stormy Night
# brms version: 2.10.0