Is there already an extension to knitr/Rmarkdown that instead of 'warning=FALSE' lets you specify which warnings should be suppressed/ignored (and thus, which others not)?

I want to take precautions against code rot in a very large and possibly long-lived document.

This requires some refinement, but how about modifying the knitr::knit_hooks for "warnings" to search each warning for known patterns and only printing those that aren't found e.g.

---
title: "Untitled"
author: "Mike Smith"
date: "12/8/2021"
output: html_document
---

```{r setup, include=FALSE}
knitr::knit_hooks$set(warning = function(x, options) { 

    if(options$warning == TRUE) {
        sprintf('<div class="warning"><pre class="knitr %s">%s</pre></div>\n', tolower(options$engine), x)
    } else {
        known_warning <- any(vapply(options$warning, 
                                FUN = grepl, 
                                FUN.VALUE = logical(1), 
                                x = x))
        if(!known_warning) {
            sprintf('<div class="warning"><pre class="knitr %s">%s</pre></div>\n', tolower(options$engine), x)
        }
    }

})
```

## Default warning handling

```{r, warning = TRUE}
1:2 + 3:5
```

## Suppress known warning

```{r, warning = c("longer object length")}
1:2 + 3:5
```

## Suppress known warning, but print new warning

```{r, warning = c("longer object length")}
1:2 + 3:5
testit <- function() warning("testit")
testit()
```

which renders to:

Thank you, Mike! Here, in case anyone else is interested, is the code I am now using for the MSMB book. It addresses the fact that both x and options$warning can be vectors, and cleans up gratuitous formatting inserted by knit.

cleanup = function(x) gsub("\n#*", "", gsub("^## ", "", x))

knitr::knit_hooks$set(warning = function(x, options) {
  ow   = options$warning
  x    = cleanup(unique(x)) 
  keep = if (is.character(ow) && length(ow) > 0) {
           res = rep(TRUE, length(x))
           for (w in ow)
             res = res & !grepl(w, x, ignore.case = TRUE)
           res
         } else if(is.logical(ow) && length(ow)==1) {
           rep(ow, length(x))
         } else {
           stop("In knit hook for warning, do not know how to handle 'options$warning'.")
         }

  if (any(keep))
    sprintf('<div class="warning"><pre class="knitr %s">%s</pre></div>\n', tolower(options$engine), x[keep])
})

It is then used in code chunks like in the following example.

```{r dbscanfcs, warning = c("stat_contour..: Zero contours were generated", "min.x.: no non-missing arguments to min; returning Inf", "max.x.: no non-missing arguments to max; returning -Inf")}
res5 = dbscan::dbscan(mc5, eps = 0.65, minPts = 30)
mc5df = data.frame(mc5, cluster = as.factor(res5$cluster))
ggplot(mc5df, aes(x = CD4, y = CD8,  col = cluster)) + geom_density2d()
```