A number of mods are still on 1.19.2 and are either still in the process of being updated, or won't be updated.

I myself tried to update my pack to 1.19.3 so I could add Plethora, but unfortunately we would loose too many essential mods in doing so.

Yep, as long as you follow the license you're allowed to redistribute modified versions. Though obviously you're going to get any official support - there's been a lot of bugfixes since the 1.19.3 update.

The main problem with 1.19.2 is that you need to use CC: Restitched, which has several API differences compared with CC:T (which is only available for 1.19.3). You'd also need to revert 15e3497, and possibly some other later commits.

I think that backporting the mod now is unlikely to happen, but you could try building the mod yourself using its source before it was updated to 1.19.3: dd30866

If I was to rebuild a previous commit, I'd have to provide the mod jar manually in my modpack rather than using curseforge - would that be allowed?

I'll try rebuild it this afternoon although I'm not a Java dev so it'll take me some time to figure it out.