Mvgam: an R package for time series modeling and forecasting (built with Stan) by Nicholas Clark
... Dynamic Generalized Additive Models (DGAMs) for time series with dynamic trend components. It uses a State-Space framework with a formula syntax based on that of the package mgcv to provide a familiar GAM modelling interface. There is also built-in support for the increasingly powerful marginaleffects package to make interpretation easy.
https://discourse.mc-stan.org/t/mvgam-an-r-package-for-time-series-modeling-and-forecasting/35065

