As @Tomat0@lemmy.ml says, realistically, these are long-term goals. I know of a few Discord servers that have a channel or channels dedicated to Matrix bridging, but I’ve also had admins outright refuse my overtures.

To date, my biggest coup was bridging a 500 member Telegram group with a 200 member Matrix room on the same topic. The admin on the Matrix side was sympathetic, but the Telegram admin was skeptical. It took a solid month of negotiations, with plenty of space for them to consider things and think of new questions, either about Matrix and bridging generally, or the specific initiative I was posing, and the community on the Matrix side.

There’s also the infrastructure considerations. Moderation is a big concern, particularly with Telegram, or a Discord server of significant size and / or popularity. There can be concerns about the bridge being a conduit for spam, but, equally, proposing automated solutions that bridge, like Matrix’s Mjolnir, can be a selling point. I run my own homeserver and Mjolnir instances with this in mind. Similarly, I run my own appservices, to open up bridging possibilities to more obscure IRC networks, or to take advantage of newer features than are available on the older branches run by established services like the fab t2bot.io.

In summary, it’s the long game, and spending time getting to know any particular community is essential. They may be predisposed to the idea if you target specific topics, as Tomat0 suggested, but that’s no guarantee. In fact, it could easily swing the opposite way, where concerns about logging and search engine indexing could scupper the initiative. Ultimately, successful bridges are partnerships, the criteria for which are common across many subject areas, including those well beyond technology.