I think Discord messages can be unbounded in size

Interesting, trying it out on Discord the default max msg size is 2000 chars and with Discord Nitro the max is raised to 4000 chars.

Testing with Postgres, a 2000 char random sequence doesn't result in TOASTing, but a 4000 random sequence does get TOASTed

And for kicks, 4000 chars that aren't random compress well enough that they don't end up in TOAST.

Default for TOAST is 2KB and how many chars you can fit in this space depends on encoding, could be as low as 1000 uncompressed.

I think this limit can be adjusted, if you know that your message limit is 4000 chars its definitely a good idea.