My domain name provider doesn't allow CAA records in their DNS editor for some reason. When I mailed them about it they said I could just use a TXT record instead. After a good hour of Googling this appears to have been incorrect. I can't find any information about using a TXT record for this. I have even skimmed through the RFC and I can't find any mention of TXT records.
Is a TXT record commonly supported for CAA, or was my domain name provider straight up wrong about that?
"Never attribute to malice that which is adequately explained by stupidity."
I would guess that your domain provider didn't purposefully lie to you so much as misunderstand what you were asking for because they didn't know what a CAA record was. They probably saw the word certificate somewhere in the conversation and assumed you were talking about the validation records you might need to create to prove domain ownership.
In any case, no. You can't use a TXT record in place of a CAA record to limit which CAs are allowed to issue certs for your domain.
If you like your domain registrar, you can likely move just the DNS hosting to another provider that supports CAA records. Or if this is the proverbial straw that broke the camel's back, transferring to a different registrar is also an option.