Add footer to section's start/title page

Question

I am creating a RMarkdown template of Beamer slides and use the metropolis theme as a basis. This already includes:

• Numbered TOC
• Hiding of footer on title page

Now I want to add the footer to the section's start/title pages (the page which includes only the title of the next section in the middle of the slide). I tried to show the footer on this pages by this code snipet

  - |
    \makeatletter
    \def\ps@sectionpage{%
      \setbeamertemplate{frame footer}{\tiny{\textcolor{beaublue}{Conference 56. Jahrestagung der DGSMP, 2021 | SIMON RESS}}}
    }
    \addtobeamertemplate{section page}{\thispagestyle{sectionpage}}{}
    \makeatother

, but it is not working.

---

This is my (minimal working example):

slides.rmd

---
title: "Title"
subtitle:  "Subtitle"
author: "Simon"
institute: "RUB"
date: "September 22, 2021"

output:
  beamer_presentation:
    keep_md: true
    keep_tex: no
    latex_engine: xelatex
    #theme: metropolis
    slide_level: 2 # which header level should be printed as slides
    incremental: no
header-includes:
  - \usetheme[numbering=fraction]{metropolis}
  - \definecolor{beaublue}{rgb}{0.74, 0.83, 0.9}
  - \setbeamertemplate{frame footer}{\tiny{\textcolor{beaublue}{Conference 56. Jahrestagung der DGSMP, 2021 | SIMON RESS}}}
#hide footer on title page:
  - |
    \makeatletter
    \def\ps@titlepage{%
      \setbeamertemplate{footline}{}
    }
    \addtobeamertemplate{title page}{\thispagestyle{titlepage}}{}
    \makeatother
#show footer on section's start/title pages
  - |
    \makeatletter
    \def\ps@sectionpage{%
      \setbeamertemplate{frame footer}{\tiny{\textcolor{beaublue}{Conference 56. Jahrestagung der DGSMP, 2021 | SIMON RESS}}}
    }
    \addtobeamertemplate{section page}{\thispagestyle{sectionpage}}{}
    \makeatother
#add secrtion numbers to TOC:
  - |
    \setbeamertemplate{section in toc}{
    \leavevmode%
    \inserttocsectionnumber. 
    \inserttocsection\par%
    }
    \setbeamertemplate{subsection in toc}{
    \leavevmode\leftskip=2.5em\inserttocsubsection\par}
---

```{r setup, include=FALSE}
knitr::opts_chunk$set(echo = FALSE)
```


## Content
\tableofcontents[]

# Level I

Test

## Slide with Bullets

- Bullet 1
- Bullet 2
- Bullet 3

# Slide with R Output

```{r cars, echo = TRUE}
summary(cars)
```

Answer 1

The metropolis theme uses hard coded plain,c,noframenumbering for their section pages, but you can overwrite their definition like this:

---
title: "Title"
subtitle:  "Subtitle"
author: "Simon"
institute: "RUB"
date: "September 22, 2021"

output:
  beamer_presentation:
    keep_md: true
    keep_tex: true
    latex_engine: xelatex
    #theme: metropolis
    slide_level: 2 # which header level should be printed as slides
    incremental: no
header-includes:
  - \usetheme[numbering=fraction]{metropolis}
  - \definecolor{beaublue}{rgb}{0.74, 0.83, 0.9}
  - \setbeamertemplate{frame footer}{\tiny{\textcolor{beaublue}{Conference 56. Jahrestagung der DGSMP, 2021 | SIMON RESS}}}
#hide footer on title page:
  - |
    \makeatletter
    \def\ps@titlepage{%
      \setbeamertemplate{footline}{}
    }
    \addtobeamertemplate{title page}{\thispagestyle{titlepage}}{}
    \makeatother
#show footer on section's start/title pages
  - |
    \makeatletter
    \def\ps@sectionpage{%
      \setbeamertemplate{frame footer}{\tiny{\textcolor{beaublue}{Conference 56. Jahrestagung der DGSMP, 2021 | SIMON RESS}}}
    }
    \addtobeamertemplate{section page}{\thispagestyle{sectionpage}}{}
    
    \makeatother
#add secrtion numbers to TOC:
  - |
    \setbeamertemplate{section in toc}{
    \leavevmode%
    \inserttocsectionnumber. 
    \inserttocsection\par%
    }
    \setbeamertemplate{subsection in toc}{
    \leavevmode\leftskip=2.5em\inserttocsubsection\par}
    \makeatletter
    \renewcommand{\metropolis@enablesectionpage}{
      \AtBeginSection{
        \ifbeamer@inframe
          \sectionpage
        \else
          \frame[c]{\sectionpage}
        \fi
      }
    }
    \metropolis@enablesectionpage
    \makeatother
---

```{r setup, include=FALSE}
knitr::opts_chunk$set(echo = FALSE)
```


## Content
\tableofcontents[]

# Level I

Test

## Slide with Bullets

- Bullet 1
- Bullet 2
- Bullet 3

# Slide with R Output

```{r cars, echo = TRUE}
summary(cars)
```

---