There is a dedicated DNS SPF record which can be used for SPF record. However DNS TXT record may also be used for backwards compatibility.
If i CAN define SPF record - should i just delete the deprecated TXT record or leave it?
I chose to leave mine. It doesn't cost anything, and may continue to work.
Revisit this in a few years when more servers/clients have updated, and if you can, see who is asking for the TXT record perhaps.