I think I figured out that the service that I was planning on using (i.e. forwardemail DOT net) was using one kind of TXT record as a proprietary means of constraining how forwarding is done. Or, it’s a way of driving users towards paid plans. (If either of these explanations is right, then I suppose that even though SPF record can be considered a special kind of TXT record, for this specific TXT record it is not really a standard SPF record.)
Following is an address with instructions that I was following; in particular, I was looking at step four and option A:
(I also add an SPF record that looks more standard as part of step five: “v=spf1 a mx include:spf.forwardemail.net -all”.)
I have noticed that upon switching to improvmx DOT com, they do not ask me to add a TXT record involving “forward-email” and an at-sign. I’m not entirely sure if this comes at a cost of being more prone to having my emails be tagged as spam, however.