Error when using bslib component in Bootstrap 4 Shiny app

Describe the problem

Hi,

After upgrading the bslib package to the latest version (0.9.0), our app throws an error when I try to specify Bootstrap v4 in Shiny UI and use a bslib component like nav_spacer.
The error message is: 'Error in component_dependency_sass: bslib components require Bootstrap 5 or higher.'

However, it did work before we upgraded the bslib package (when it was version 0.5.1).

I'm wondering if the latest bslib package no longer supports older Bootstrap versions or it's a compatibility issue?

Here is a simple example code:

library(shiny)
library(bslib)

ui <- fluidPage(
  
  theme = bslib::bs_theme(version = 4),
  
  shiny::navbarPage(
    title = "Example",
    shiny::tabPanel(
      title = "Tab 1",
      value = "tab1"
    ),
    bslib::nav_spacer(),
    shiny::tabPanel(
      title = "Tab 2",
      value = "tab2"
    )
  )
)

server <- function(input, output, session) {
  
}

shinyApp(ui, server)

Session Info

Place your devtools::session_info() here

Start by running the minimal R example with bslib 0.9.0 and inspect the Bootstrap-version handling reached by nav_spacer() and component_dependency_sass. Compare the behavior with bslib 0.5.1; done means the compatibility behavior is corrected or the supported Bootstrap versions are clearly documented and covered by a regression test.