White vinegar might be the best replacement for fabric softener
Healthy Home Swaps : Fabric Softener - Clean Mama
How to remove the vinegar smell from fabrics that were washed with vinegar and liquid detergent but not rinsed well before adding fabric softener - Quora
Can You Use Vinegar Instead of Fabric Softener?
When Should You Avoid Using Vinegar In Laundry? - Downy FAQ | Downy
This or that: Vinegar vs. Fabric Softener - Figgy & Co.