Question: Is it possible to downgrade the thorium mod to make it compatible with 1.3.4.4?
1-no.
2-unless you have the previous version of thorium for modloader 9.2.3 maybe but...
3-even if you play with the previous version of thorium you cant have/play as the bard class.
Slight amendment to that...
Possible? Yes.
Feasible? No.
It's not as simple as just changing one or two things. You would basically have to take an older version of Thorium and add all the new content to it, using the methods of a 1.3.4.4 version of tModloader to accomplish them. If anything in the new Thorium depends on something new to the current version of vanilla Terraria, you'd need to either mod that in as well, or change the way it works to use what was available back then. In other words, you'd basically be re-making all the new content more or less from scratch, and you'd miss out on the bug fixes since then as well unless you went through and fixed them all yourself, too. If anything requires a feature from a newer version of tModloader, you'd have to create a "demake" version of
that too, in order to have those features in an older version, which would then necessitate also making special versions of any other mods you're using to be compatible with the altered demake version of tModloader, or else somehow cludging Thorium to have those features self-contained (which might
still cause compatibility issues with other mods).
tl;dr - Yes, obviously you
can re-create anything that has been created, but the amount of effort needed would more than likely be greater than it's worth.
Edit: In other words, unless you're content with just playing an older version of the mod, by the time you'd finished making it all work in 1.3.3.4, you'd have put in far more work than to just update to the current versions of everything, and for largely the same result. My guess, however, is that you're wanting to play alongside mods that haven't updated to the newest version. If so, I'd recommend that, rather than trying to backport Thorium, you learn how to update them yourself if you really just can't wait. It's generally considered in bad taste to put out a version of someone else's mod without their permission, yes. If you're just making a personal version for your own private use, though, that's nobody's business but yours.