Not sure of anytghing that will do quite what you want but here are two lists for "everything" on Steam that is tagged as a demo.

https://completionist.me/steam/apps?display=grid&order=desc&sort=created&search=&type=demo
https://steamdb.info/search/?a=app&q=&type=3&category=0

Run this script (F12 > console) over at your wishlist page:

var demos = {};
var n = Object.keys(g_rgAppInfo).length;
var storeurl = "https://store.steampowered.com/app/";
Object.keys(g_rgAppInfo).forEach(appid => {
jQuery.get(storeurl + appid, data => {
if (jQuery("[href*='steam://install/']", data).length) {
demos[appid] = jQuery("[href*='steam://install/']", data).attr("href").split("/")[3].split("'")[0].split(",")[0];
}
}).always(() => {
n--;
if (n <= 0) {
console.log(Object.entries(demos).map(e => storeurl + e[0] + " has demo steam://install/" + e[1]).join("\n"));
}
});
});

Enjoy!
I love doing these (small) scripts. If you need a job done, I do paid work too, please contact me!

Steamgauge