Looks like the deprecation notice on https://matrix-org.github.io/synapse/latest/spam_checker.html is easier to miss than I thought. @anoadragon453 suggested:

as far as the warning goes, I would start by putting it below "Handling spam in Synapse", then probably bumping it to a higher header number (maybe even just h1).

we can do red - none of the themes have red text as the default (yet)