It seems Discourse provides a 512px icon in the web manifest JSON file. Under site settings, branding, there is just one manifest icon field which hints that uploaded images will be scaled to 512×512px.
{
...
"icons":[
{
"src":"https://community.toph.co/uploads/default/optimized/2X/5/5b9de7676657b71384fb77c662616d73765f0293_2_512x512.png",
"sizes":"512x512",
"type":"image/png"
},
{
"src":"https://community.toph.co/uploads/default/optimized/2X/5/5b9de7676657b71384fb77c662616d73765f0293_2_512x512.png",
"sizes":"512x512",
"type":"image/png",
"purpose":"maskable"
}
],
...
}
However, according to https://web.dev/add-manifest/:
For Chrome, you must provide at least a 192x192 pixel icon, and a 512x512 pixel icon.
I noticed this is true for Firefox as well (at least on a LineageOS device). Here is what it looks:
Notice the two icons for Toph and Drafts? Both are providing 192px icons (among other sizes). Previously Toph and Drafts several icons but not 192px ones. And, they appeared similar to the icon of Community (i.e. had that dark padding around the white area). The Community app in this screenshot is a Discourse instance.